Method and apparatus for spinal distraction and fusion

ABSTRACT

The invention provides an implantable spinal distraction/fusion rod with varied thread pitch and diameters along different portions of its length that it is capable of distracting two or more vertebral bodies relative to each other and/or facilitating the procedure of fusing the vertebral bodies together from within the spine. The present invention also provides for methods of using the rod to distract and/or fuse two or more vertebral bodies from within the spine.

This application is a continuation of U.S. patent application Ser. No.11/189,943 filed Jul. 26, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/309,416 filed on Dec. 3, 2002, now U.S. Pat. No.6,921,403, which is a continuation-in-part of U.S. patent applicationSer. No. 10/125,771 filed on Apr. 18, 2002, now U.S. Pat. No. 6,899,716,which is a continuation-in-part of U.S. patent application Ser. No.09/848,556 filed on May 3, 2001, now U.S. Pat. No. 7,014,633, which is acontinuation-in-part of U.S. patent application Ser. No. 09/782,583filed on Feb. 13, 2001, now U.S. Pat. No. 6,558,390, which claimspriority to U.S. Provisional Patent Application Ser. No. 60/182,748filed on Feb. 16, 2000, the contents of each of which are incorporatedin their entirety into this disclosure by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to spinal surgery, particularlymethods and apparatus for forming one or more trans-sacral axial spinalinstrumentation/fusion (TASIF) axial bore through vertebral bodies ingeneral alignment with a visualized, trans-sacral anterior or posterioraxial instrumentation/fusion line (AAIFL or PAIFL) in a minimallyinvasive, low trauma, manner and providing a therapy to the spineemploying the axial bore.

It has been estimated that 70% of adults have had a significant episodeof back pain or chronic back pain emanating from a region of the spinalcolumn or backbone. Many people suffering chronic back pain or an injuryrequiring immediate intervention resort to surgical intervention toalleviate their pain.

The spinal column or backbone encloses the spinal cord and consists of33 vertebrae superimposed upon one another in a series which provides aflexible supporting column for the trunk and head. The vertebraecephalad (i.e., toward the head or superior) to the sacral vertebrae areseparated by fibrocartilaginous intervertebral discs and are united byarticular capsules and by ligaments. The uppermost seven vertebrae arereferred to as the cervical vertebrae, and the next lower twelvevertebrae are referred to as the thoracic, or dorsal, vertebrae. Thenext lower succeeding five vertebrae below the thoracic vertebrae arereferred to as the lumbar vertebrae and are designated L1-L5 indescending order. The next lower succeeding five vertebrae below thelumbar vertebrae are referred to as the sacral vertebrae and arenumbered S1-S5 in descending order. The final four vertebrae below thesacral vertebrae are referred to as the coccygeal vertebrae. In adults,the five sacral vertebrae fuse to form a single bone referred to as thesacrum, and the four rudimentary coccyx vertebrae fuse to form anotherbone called the coccyx or commonly the “tail bone”. The number ofvertebrae is sometimes increased by an additional vertebra in oneregion, and sometimes one may be absent in another region.

Typical lumbar, thoracic and cervical vertebrae consist of a ventral orvertebral body and a dorsal or neural arch. In the thoracic region, theventral body bears two costal pits for reception of the head of a rib oneach side. The arch which encloses the vertebral foramen is formed oftwo pedicles and two lamina. A pedicle is the bony process whichprojects backward or anteriorly from the body of a vertebra connectingwith the lamina on each side. The pedicle forms the root of thevertebral arch. The vertebral arch bears seven processes: a dorsalspinous process, two lateral transverse processes, and four articularprocesses (two superior and two inferior). A deep concavity, inferiorvertebral notch, on the inferior border of the arch provides apassageway or spinal canal for the delicate spinal cord and nerves. Thesuccessive vertebral foramina surround the spinal cord. Articulatingprocesses of the vertebrae extend posteriorly of the spinal canal.

The bodies of successive lumbar, thoracic and cervical vertebraearticulate with one another and are separated by the intervertebralspinal discs. Each spinal disc comprises a fibrous cartilage shellenclosing a central mass, the “nucleus pulposus” (or “nucleus” herein)that provides for cushioning and dampening of compressive forces to thespinal column. The shell enclosing the nucleus comprises cartilaginousendplates adhered to the opposed cortical bone endplates of the cephaladand caudal vertebral bodies and the “annulus fibrosis” (or “annulus”herein) comprising an annular fibrosis layer of collagen fibers runningcircumferentially around the nucleus pulposus and connecting thecartilaginous endplates. The nucleus contains hydrophilic (waterattracting) mucopolysacharides and fibrous strands. The nucleus isrelatively inelastic, but the annulus can bulge outward slightly toaccommodate loads axially applied to the spinal motion segment.

The intervertebral discs are anterior to the spinal canal and locatedbetween the opposed end faces or endplates of a cephalad and a caudalvertebral body. The inferior articular processes articulate with thesuperior articular processes of the next succeeding vertebra in thecaudal (i.e., toward the feet or inferior) direction. Several ligaments(supraspinous, interspinous, anterior and posterior longitudinal, andthe ligamenta flava) hold the vertebrae in position yet permit a limiteddegree of movement. The assembly of two vertebral bodies, theinterposed, intervertebral, spinal disc and the attached ligaments,muscles and facet joints is referred to as a “spinal motion segment”.

The relatively large vertebral bodies located in the anterior portion ofthe spine and the intervertebral discs provide the majority of theweight bearing support of the vertebral column. Each vertebral body hasrelatively strong, cortical bone layer comprising the exposed outsidesurface of the body, including the endplates, and weak, cancellous bonecomprising the center of the vertebral body.

A number of spinal disorders are caused by traumatic spinal injuries,disease processes, aging processes, and congenital abnormalities thatcause pain, reduce the flexibility of the spine, decrease the loadbearing capability of the spine, shorten the length of the spine, and/ordistort the normal curvature of the spine. These spinal disorders andvarious treatments that have been clinically used or proposed are firstdescribed as follows.

With aging, the nucleus becomes less fluid and more viscous andsometimes even dehydrates and contracts (sometimes referred to as“isolated disc resorption”) causing severe pain in many instances. Inaddition, the annulus tends to thicken, desiccate, and become morerigid, lessening its ability to elastically deform under load and makingit susceptible to fracturing or fissuring.

One form of degeneration of the disc occurs when the annulus fissures oris rent. The fissure may or may not be accompanied by extrusion ofnucleus material into and beyond the annulus. The fissure itself may bethe sole morphological change, above and beyond generalized degenerativechanges in the connective tissue of the disc, and disc fissures cannevertheless be painful and debilitating. Biochemicals contained withinthe nucleus are allowed to escape through the fissure and irritatenearby structures.

A fissure also may be associated with a herniation or rupture of theannulus causing the nucleus to bulge outward or extrude out through thefissure and impinge upon the spinal column or nerves (a “ruptured” or“slipped” disc). With a contained disc herniation, the nucleus may workits way partly through the annulus but is still contained within theannulus or beneath the posterior longitudinal ligament, and there are nofree nucleus fragments in the spinal canal. Nevertheless, even acontained disc herniation is problematic because the outward protrusioncan press on the spinal cord or on spinal nerves causing sciatica.

Another disc problem occurs when the disc bulges outwardcircumferentially in all directions and not just in one location. Thisoccurs when over time, the disc weakens, bulges outward and takes on a“roll” shape. Mechanical stiffness of the joint is reduced and thespinal motion segment may become unstable shortening the spinal cordsegment. As the disc “roll” extends beyond the normal circumference, thedisc height may be compromised, and foramina with nerve roots arecompressed causing pain. In addition, osteophytes may form on the outersurface of the disc roll and further encroach on the spinal canal andforamina through which nerves pass. The cephalad vertebra may eventuallysettle on top of the caudal vertebra. This condition is called “lumbarspondylosis”.

In addition, various types of spinal column displacement disorders areknown in one or more spinal motion segment that are hereditary or arecaused by degenerative disease processes or trauma. Such spinaldisplacement disorders include scoliosis (abnormal lateral curvature ofthe spine), kyphosis (abnormal forward curvature of the spine, usuallyin the thoracic spine), excess lordosis (abnormal backward curvature ofthe spine, usually in the lumbar spine), spondylolisthesis (forwarddisplacement of one vertebra over another, usually in the lumbar orcervical spine). At times the displacement disorder is accompanied by orcaused by a fracture or partial collapse of one or more vertebrae ordegeneration of a disc. Patients who suffer from such conditions canexperience moderate to severe distortion of the thoracic skeletalstructure, diminished ability to bear loads, loss of mobility, extremeand debilitating pain, and oftentimes suffer neurologic deficit in nervefunction.

Approximately 60% of spinal surgery takes place in the lumbar spine, andof that portion approximately 80% involves the lower lumbar vertebraedesignated as the fourth lumbar vertebra (“L4”), the fifth lumbarvertebra (“L5”), and the first sacral vertebra (“S1”). Persistent lowback pain is attributed primarily to degeneration of the disc connectingL5 and S1. Traditional, conservative methods of treatment include bedrest, pain and muscle relaxant medication, physical therapy or steroidinjection. Upon failure of conservative therapy spinal pain hastraditionally been treated by spinal fusion, with or withoutinstrumentation, which causes the vertebrae above and below the disc togrow solidly together and form a single, solid piece of bone.

Highly invasive, open surgical procedures have been developed and usedto perform a “complete discectomy” to surgically remove the disc, andthe vertebral bodies are then fused together. The removal of the discinvolves removing the nucleus, cutting away the cartilaginous endplatesadhered to the opposed cortical bone endplates of the cephalad andcaudal vertebral bodies, and removing at least a portion of the annulus.Fusion of the vertebral bodies involves preparation of the exposedendplate surfaces by decortication (scraping the endplate cortical bone)and the deposition of additional bone into disc space between theprepared endplate surfaces. The complete discectomy and fusion may beperformed through a posterior surgical route (from the back side of thepatient) or an anterior surgical route (from the front side of thepatient). The removed vertebral bone may be just the hard cortical boneor may include soft cancellous soft bone in the interior of thevertebral bodies. Controversy exists regarding the preferred method ofperforming these fusions for various conditions of the spine. Sometimes,non-biological materials are used to augment and support the bone grail(fixation systems). Sometimes, the fixation is performed from theposterior route (posterior fixation), or from the anterior route(anterior fixation), or even both sides (anterior-posterior fixations orcircumferential fusion).

Current treatment methods other than spinal fusion for symptomatic discrolls and herniated discs include “laminectomy” which involves thesurgical exposure of the annulus and surgical excision of thesymptomatic portion of the herniated disc followed by a relativelylengthy recuperation period.

Various other surgical treatments that attempt to preserve theintervertebral spinal disc and to simply relieve pain include a“nucleotomy” or “disc decompression” to remove some or most of theinterior nucleus thereby decompressing and decreasing outward pressureon the annulus. In less invasive microsurgical procedures known as“microlumbar discectomy” and “automated percutaneous lumbar discectomy”,the nucleus is removed by suction through a needle laterally extendedthrough the annulus. Although these procedures are less invasive thanopen surgery, they nevertheless suffer the possibility of injury to thenerve root and dural sac, perineural scar formation, reherniation of thesite of the surgery, and instability due to excess bone removal.Moreover, they involve the perforation of the annulus.

Another method of treatment is known as “chemonucleolysis”, which iscarried out by injection of the enzyme chymopapain into the nucleusthrough the annulus. This procedure has many complications includingsevere pain and spasm, which may last up to several weeks followinginjection. Sensitivity reactions and anaphylactic shock occur in limitedbut significant numbers of patients.

Although damaged discs and vertebral bodies can be identified withsophisticated diagnostic imaging, the surgical procedures are soextensive that clinical outcomes are not consistently satisfactory.Furthermore, patients undergoing such fusion surgery experiencesignificant complications and uncomfortable, prolonged convalescence.Surgical complications include disc space infection, nerve root injury,hematoma formation, and instability of adjacent vertebrae.

Many surgical techniques, instruments and spinal disc implants have beendescribed in the medical literature and in patents that are directed toproviding less invasive, percutaneous, lateral access to a degeneratedintervertebral spinal disc. Then, instruments are introduced throughlateral disc openings made through the annulus for performing adiscectomy and implanting bone growth materials or biomaterials orspinal disc implants inside the annulus. Or, one or more laterallyextending space or hole is bored through the disc to receive one or morelaterally inserted spinal disc implant or bone growth material topromote fusion or to receive a pre-formed, artificial, functional discreplacement implant as typified by U.S. Pat. Nos. 5,700,291.

Percutaneous lateral procedures and instruments for performing suchdiscectomies are disclosed in U.S. Pat. Nos. Re. 33,258, 4,573,448,5,015,255, 5,313,962, 5,383,884, 5,702,454, 5,762,629, 5,976,146,6,095,149, and 6,127,597 and in PCT publication WO 99/47055, forexample. A laparascopic technique and apparatus for traversing theretroperitoneal space from an abdominal skin incision to an anteriorsurface of the disc annulus and performing a discoscopy is disclosed inthe '962 patent, for example. Percutaneous surgical disc procedures andapparatus that accesses the disc in a posterolateral approach from askin incision in the patient's back are described in the '629 and '448patents, for example.

The nucleus is fragmented by various mechanical cutting heads asdisclosed, for example in the '258, '962, '884, and '597 patents, forexample. Or, thermal or laser energy is applied to desiccate the nucleusand to stiffen the annulus as described in the '149 patent, for example.Or, the nucleus and portions of the cephalad and caudal vertebral bodiesare mechanically cut away to enlarge the disc space as described in thePCT '055 publication and in the '255 patent, for example. Irrigationfluid is introduced into the disc space or cavity and the fragments ordesiccation by-products of the nucleus and any bone and annulusfragments are aspirated from the disc space or cavity. The irrigationand aspiration is effected through an access cannula positioned againstthe opening through the annulus of the herniated disc as disclosed inthe '629 patent, for example, or through a lumen of the discectomyinstrument, as disclosed in the '258 patent, for example. A measure ofsafety and accuracy is added to these operative procedures by theartiroscopic visualization of the annulus and other important structureswhich lie in the path of the instruments, such as the spinal nerve.

The above-described procedures involve invasive surgery that laterallyexposes the anterior or posterior (or both) portions of the vertebraeand intervertebral spinal disc. Extensive muscular stripping and bonepreparation can be necessary. As a result, the spinal column can befurther weakened and/or result in surgery induced pain syndromes. Thus,presently used or proposed surgical fixation and fusion techniquesinvolving the lower lumbar vertebrae suffer from numerous disadvantages.

Methods and apparatus for accessing the discs and vertebrae by lateralsurgical approaches that purportedly reduce muscular stripping (and thatare similar to those disclosed in the above-referenced '629 and '888patents) are described in U.S. Pat. No. 5,976,146. The interveningmuscle groups or other tissues are spread apart by a cavity forming andsecuring tool set disclosed in the '146 patent to enable endoscopeaided, lateral access to damaged vertebrae and discs and to performcorrective surgical procedures. However, it is preferable to avoid thelateral exposure to correct less severe spondylolisthesis and otherspinal injuries or defects affecting the lumbar and sacral vertebrae anddiscs.

A less intrusive posterior approach for treating spondylolisthesis isdisclosed in U.S. Pat. No. 6,086,589, wherein a straight bore is formedthrough the sacrum from the exposed posterior sacral surface and in aslightly cephalad direction into the L5 vertebral body, preferably afterrealigning the vertebrae. A straight, hollow, threaded shaft with sidewall holes restricted to the end portions thereof and bone growthmaterial are inserted into the bore. A discectomy of the disc between L5and S1 is preferably performed in an unexplained manner, and boneingrowth material is also preferably inserted into the space between thecephalad and caudal vertebral bodies. Only a limited access to andalignment of S1 and L5 can be achieved by this approach because thedistal ends of the straight bore and shaft approach and threaten toperforate the anterior surface of the L5 vertebral body. This approachis essentially a posteriolateral approach that is intended to fuse S1and L5 and cannot access more cephalad vertebral bodies orintervertebral spinal discs.

In many of these procedures, a laterally extending space is prepared byremoval of the disc to receive one or more disc implant, and insertionof a bone growth material, e.g. autologous bone, or a pre-formed,artificial, functional disc replacement implant. A number of discshaped, functional disc replacement implants and methods of insertionhave been proposed as disclosed, for example, in U.S. Pat. Nos.5,258,031 and 6,019,792, for example. Other disc shaped or vertebralbody replacement implants that are designed to encourage bone growth andeffect fusion are shown in U.S. Pat. Nos. 5,514,180 and 5,888,223, forexample. These devices and techniques are intended to overcome thedisadvantages of purely surgical techniques to mechanically immobilizeareas of the spine assisting in the eventual fusion of the treatedadjacent vertebrae, and to maintain the length of the treated spinalmotion segment to avoid shortening spinal cord and nerve segments.However, they require relatively large lateral exposure of the disc orvertebral body to excise the disc or vertebral body, shape the adjoiningcaudal and cephalad vertebral bodies and effect the implantation andfixation thereto. Thus, disadvantages to the present implants andsurgical implantation techniques remain concerning the implantationprocedures and involving post-surgical failure necessitatingre-operation.

A further type of disc implant that has been clinically employed forspinal fusion comprises a hollow, cylindrical, titanium cage that isexternally threaded and is screwed transversely into place in a lateralbore formed through the disc between two adjacent vertebrae. Typically,the bore involves discectomy of the damaged disc and removal of portionsof the cortical bone of the adjoining vertebral bodies to prepare atransversely extending space to receive one or more disc implant. Bonegrafts from cadavers or the pelvis or substances that promote bonegrowth are then packed into the hollow center of the cage to encouragebone growth (or ingrowth) through the cage pores to achieve fusion ofthe two adjacent vertebrae. Two such cage implants and the surgicaltools employed to place them are disclosed in U.S. Pat. Nos. 5,505,732and 5,700,291, for example. The cage implants and the associatedsurgical tools and approaches require precise drilling of a relativelylarge hole for each such cage between two adjacent vertebral bodies andthen threading a cage into each prepared hole. The exposed ends of thecage or side by side installed cages can irritate nerves causing pain toemerge again.

These approaches involve a virtually complete discectomy of the discachieved by instruments introduced laterally through the patient's bodyto the disc site and manipulated to cut away or drill lateral holesthrough the disc and adjoining cortical bone. The large laterallydrilled hole or holes can compromise the integrity of the vertebralbodies, and the spinal cord can be injured if they are drilled tooposteriorly. The endplates of the vertebral bodies, which comprise veryhard cortical bone and help to give the vertebral bodies neededstrength, are usually weakened or destroyed during the drilling. Thecylindrical cage or cages are now harder than the remaining bone of thevertebral bodies, and the vertebral bodies tend to collapse or“telescope” together. The telescoping causes the length of the vertebralcolumn to shorten and can cause damage to the nerve root and nerves thatpass between the two adjacent vertebrae.

Therefore, it is often necessary to also mechanically stabilize thevertebrae on either side of the spinal disc that is augmented or removedso that fusion of the vertebral bodies can occur successfully withouttelescoping of the vertebral bodies or movement of the disc implants outof the prepared site. One technique for spinal fixation includes theimmobilization of the spine by the use of spine rods of many differentconfigurations that run generally parallel to the spine. Typically, theposterior surface of the spine is isolated and bone screws are firstfastened to the pedicles of the appropriate vertebrae or to the sacrumand act as anchor points for the spine rods. The bone screws aregenerally placed two per vertebra, one at each pedicle on either side ofthe spinous process. Clamp assemblies join the spine rods to the screws.The spine rods are generally bent to achieve the desired curvature ofthe spinal column. Wires may also be employed to stabilize rods tovertebrae. These techniques are described further in U.S. Pat. No.5,415,661, for example.

These types of rod systems can be effective, but require a posteriorapproach and implanting screws into or clamps to each vertebra over thearea to be treated. To stabilize the implanted system sufficiently, onevertebra above and one vertebra below the area to be treated are oftenused for implanting pedicle screws. Since the pedicles of vertebraeabove the second lumbar vertebra (L2) are very small, only small bonescrews can be used which sometimes do not give the needed support tostabilize the spine. These rods and screws and clamps or wires aresurgically fixed to the spine from a posterior approach, and theprocedure is difficult. A large bending moment is applied to such rodassemblies, and because the rods are located outside the spinal column,they depend on the holding power of the associated components which canpull out of or away from the vertebral bone.

In a further approach disclosed in U.S. Pat. Nos. 4,553,273 and4,636,217, both described in U.S. Pat. No. 5,735,899, two of threevertebrae are joined by surgically obtaining access to the interior ofthe upper and lower vertebral bodies through excision of the middlevertebral body. In the '899 patent, these approaches are referred to as“intraosseous” approaches, although they are more properly referred toas “interosseous” approaches by virtue of the removal of the middlevertebral body. The removal is necessary to enable a lateral insertionof the implant into the space it occupied so that the opposite ends ofthe implant can be driven upward and downward into the upper and lowervertebral bodies. These approaches are criticized as failing to provideadequate medial-lateral and rotational support in the '899 patent. Inthe '899 patent, an anterior approach is made, slots are created in theupper and lower vertebrae, and rod ends are fitted into the slots andattached to the remaining vertebral bodies of the upper and lowervertebrae by laterally extending screws. These approaches involveconsiderable damage to ligaments and tissue in the anterior access tothe vertebral bones.

The use of radiopaque metal cages or other metal implants also makes itdifficult to image the disc space with radiographic imaging equipment toassess the degree of fusion achieved by bone growth between thevertebral bodies separated by the cages. Laterally insertable, rigidcarbon fiber and more flexible polymeric disc implants are under studyas replacements for metal implants.

Alternatively, the use of a deflated porous fabric bag that is laterallyinserted into a prepared cavity and inflated with bone growthencouraging material is disclosed in U.S. Pat. No. 5,549,679. Theprepared cavity is substantially ovaloid and includes the removed discand a portion of the adjoining vertebral bodies. The filling of the bagunder pressure tends to distract, i.e., to separate, the adjoiningvertebral bodies to the physiologic separation that would be provided bythe undamaged disc. The porous bag opening is closed in a number of waysto retain the material it is filled with. This porous bag isdistinguished from several other artificial disc designs described inthe '679 patent, including an artificial disc with an elastomeric core(U.S. Pat. No. 5,071,437) or filled with hydrogel beads (U.S. Pat. No.5,192,326).

In a further disc augmentation approach described in U.S. Pat. No.5,888,220, the disc is accessed laterally through the patient's body,the annulus is perforated unless it is already rent, and a partialdiscectomy is performed to remove most or all of the nucleus to create aspace within the annulus. Then, a mass of curable biomaterials isinjected into the prepared space and the material is cured in situ. Inone variation, a deflated balloon is inserted into the prepared space,and the mass of curable biomaterials is injected into the prepared spaceand the material is cured in situ, leaving the filled balloon andsolidified biomaterial in place.

A compilation of many of the above described surgical techniques andspinal implants and others that have been used clinically is set forthin certain chapters of the book entitled Lumbosacral and SpinopelvicFixation, edited by Joseph Y. Margolies et al. (Lippincott-RavenPublishers, Philadelphia, 1996). Attention is directed particularly toChapters 1, 2, 17, 18, 38, 42 and 44.

In “Lumbopelvic Fusion” (Chapter 38, by Prof. Rene P. Louis, MD)techniques for repairing a spondylolisthesis, in this case, a severedisplacement of L5 with respect to S1 and the intervening disc, aredescribed and depicted. An anterior lateral exposure of L5 and S1 ismade, a discectomy is performed, and the orientation of L5 to S1 ismechanically corrected using a reduction tool, if the displacement issevere. A fibula graft or metal Judet screw is inserted as a dowelthrough a bore formed extending caudally through L5 and into S1. Whenthe screw is used, bone growth material, e.g., bone harvested from thepatient, is inserted into the bore alongside the screw, and the discspace is filled with bone sutured to the screw to keep it in placebetween the vertebral surfaces to act as a spacer implant occupying theextracted disc between L5 and S1. External bridge plates or rods arealso optionally installed. The posterolateral or anterior lateralapproach is necessitated to correct the severe spondylolisthesisdisplacement using the reduction tool and results in tissue injury.Because of this approach and need, the caudal bore and inserted theJudet screw can only traverse L5 and S1.

A similar anterior approach for treating spondylolisthesis is disclosedin U.S. Pat. No. 6,056,749. In this approach, a bore hole is formed in acephalad vertebral body and extends through the intervening disc into acaudal vertebral body, the disc is removed, a disc cage is insertedlaterally into the disc space, and an elongated, hollow threaded shaftis inserted into the bore and through a hole in the disc cage. The disccage takes the place of the harvested bone disc inserts and itsinterlocking intersection with the shaft takes the place of the suturesemployed to tie the harvested bone disc inserts to the screw in thetechnique described in the above-referenced Chapter 38 publication.

Turning to a further spinal disorder, the vertebral bodies can thin andweaken with the development and progression of osteoporosis and certaineating disorders to the point that one or more vertebral bodycompression fractures occur as described in U.S. Pat. Nos. 4,969,888,5,972,015 and 6,066,154. Vertebral compression fractures of healthyvertebral bodies can also occur due to injury. In severe cases, thevertebral body tends to collapse, shortening the vertebral body and thespine and inducing an aberrant localized spinal curvature. As noted inthe '888 patent, osteoporotic vertebral body compression fractures arecurrently treated with bed rest, analgesics, and intravenous hydrationduring the first week after onset of the problem. These steps arefollowed by the prescription of a soft or firm spinal corset, dependingupon the physician's preference. In most cases, the corset is not wornbecause the patient suffers much discomfort and oftentimes greaterdiscomfort than that due to the fracture of the vertebral body. Thefracture pain lasts from two to eight months. In many cases, patientswith osteoporotic vertebral body collapse fractures require about oneweek in an acute care hospital and two to three weeks in an extendedcare facility until they are able to move about independently and withonly moderate pain. Current treatment does not substantially alter theconditions of the vertebral body.

The '888 patent describes a “balloon-assisted vertebroplasty” method ofrestoring the vertical height of a collapsed, compression fracturedvertebral bone through a posterolateral approach from an entry point onthe skin determined radiologically and is located approximately 10 cmfrom the midline and just inferior to a rib if present at that level. Aguide pin is extended from the incision to the vertebral body andthrough the cortical bone and a predetermined distance into thecancellous bone. A cannula is inserted over the guide pin and its distalend is attached to the exterior cortical bone of the vertebral body. Adrill is extended through the cannula and used to drill a hole into thecancellous bone to enlarge the cavity to be treated. A deflated,expandable balloon is inserted through the cannula and inflated insidethe vertebral body into a disc or checker shape. The expansion of theballoon compacts the cancellous bone against the inner surface of theouter cortical wall of the vertebral body thereby further enlarging thecavity and, it is asserted, filling the fractures in the cortical bone.The balloon expansion may also restore the height of the vertebral bodyto some extent. The balloon is then deflated and removed, and the cavityis irrigated with saline. The cavity is simultaneously aspirated andfilled with a flowable synthetic bone material or methyl methacrylatecement that is allowed to set to a hardened condition through thecannula. It is asserted that the compacted cortical bone or bone marrowwill substantially prevent flow through the fracture.

The '015 and '154 patents disclose generally the same procedure stepsbut employ improved, irregularly shaped, balloons that approximate theinner shape of the vertebral bodies they are inflated within in order tomaximally compress cancellous bone. The balloons are made of inelasticmaterial and are kept in their defined configurations when inflated byvarious shape restraints. This procedure is also referred to as a“Kyphoplasty”, by Kyphon, Inc., the assignee of the '015 and '154patents.

There are other therapeutic treatments for encouraging bone growthwithin a vertebral body or to fuse vertebral bodies together with orwithout a pre-formed spinal disc replacement implant that involveinjection of bone growth materials into the disc or vertebral body orthe application of electrical energy to stimulate bone growth. Severalnatural or artificial osteoconductive, osteoinductive, osteogenic orother fusion enhancing materials are disclosed in U.S. Pat. No.6,123,705. A system and method for delivering electrical energy to apre-formed spinal disc replacement implant to promote bone growth andfusion about the implant and between the opposed endplates of thecephalad and caudal vertebral bodies are disclosed in U.S. Pat. No.6,120,502.

A wide variety of orthopedic implants have also been proposed orclinically employed to stabilize broken bones or secure artificial hip,knee and finger joints. Frequently, rods or joint supports are placedlongitudinally within longitudinal bores made in elongated bones, e.g.,the femur. A surgical method is disclosed in U.S. Pat. No. 5,514,137 forstabilizing a broken femur or other long bones using an elongated rodand resorbable cement. To accomplish a placement of a rod into anysingle bone, an end of a bone is exposed and a channel is drilled fromthe exposed end to the other end. Thereafter, a hollow rod is inserted,and resorbable cement is injected through the hollow rod, so as toprovide fixation between the distal end of the rod and the cancelloustissue that surrounds the rod. A cement introducer device can also beused for the injection of cement. A brief reference is made in the '137patent to the possibility of placing rods in or adjacent to the spine inthe same manner, but no particular approach or devices are described.

Drilling tools are employed in many of the above described surgicalprocedures to bore straight holes into the vertebral bones. The boringof curved bores in other bones is described in U.S. Pat. Nos. 4,265,231,4,541,423, and 5,002,546, for example. The '231 patent describes anelongated drill drive shaft enclosed within a pre-curved outer sheaththat is employed to drill curved suture holding open ended bores intobones so that the suture passes through both open ends of the bore. The'423 patent describes an elongated flexible drill drive shaft enclosedwithin a malleable outer sheath that can be manually shaped into a curvebefore the bore is formed. The '546 patent describes a complex curvedrilling tool employing a pivotal rocker arm and curved guide for adrill bit for drilling a fixed curve path through bone. All of theseapproaches dictate that the curved bore that is formed follow thepredetermined and fixed curvature of the outer sheath or guide. Thesheath or guide is advanced through the bore as the bore is made, makingit not possible for the user to adjust the curvature of the bore totrack physiologic features of the bone that it traverses.

All of the above-described patents and other patents referenced hereinthat access a single spinal disc or vertebra to perform theabove-described therapies, do so from a lateral approach that involvesweakening of the spinal fusion segment. There remains a need for methodsand apparatus for performing therapeutic procedures in the spine in aminimally invasive, low trauma, manner.

SUMMARY OF THE INVENTION

There is provided in accordance with one aspect of the presentinvention, an apparatus for distracting and fusing two or more vertebralbodies. The apparatus is an implantable distraction/fusion rod that canbe used to position and fixate vertebral bodies in the human spine. Thisrod would typically be used to facilitate fusion by means of bonebridging the motion segment within which the rod is inserted. The rodcan serve many purposes, including but not limited to, modifying the gapbetween the bodies, assuming physiological axial loads, providing accessfor the introduction of osteogenic and/or osteoconductive materials.

In one embodiment of the present invention, an axially-extendingimplantable rod comprises a distal section near the leading end of therod that has threading and is capable of engaging with a first vertebralbody; a proximal section near the trailing end of the rod that hasthreading and is capable of engaging with a second vertebral body thatis located proximally relative to the first vertebral body; and anintermediate section extending between the distal and proximal sections;wherein the major and minor diameters of the threading in the proximalsection is greater than the major and minor diameters of the threadingin the distal section; wherein the thread pitch in the distal section issmaller relative to the thread pitch in the proximal section such thatadvancement of the distal section distally into the second vertebralbody causes the first and second vertebral bodies to become distractedrelative to each other.

The present invention also provides for a method of using theabove-described rod to distract and/or fuse two or more vertebral bodiesfrom within the spine.

The method of using the distraction/fusion rod generally comprises thesteps of: determining the desired change in disc height between targetedvertebral bodies; selecting a rod with the appropriate thread pitches inthe distal and proximal sections to achieve the desired change inheight; accessing the targeted bodies by creating a TASIF axial borethat extends in the distal direction from the sacral target point to thedisc space between the targeted bodies; extending the TASIF axial borein the distal direction to create an extended portion of the TASIF axialbore, wherein the extended portion has a smaller diameter than theportion of the TASIF axial bore extending from the sacral target pointto the disc space between the targeted bodies; and advancing andimplanting the selected rod into the targeted bodies to achieve thedesired change in disc height.

In accordance with another aspect of the present invention, there isprovided a method of treating the spine. The method comprises the stepsof identifying a site on the anterior surface of the sacrum. A lumen isformed from the site into the sacrum, through a first vertebral body,and into a second vertebral body. A distraction device is introducedinto the lumen.

The method additionally comprises the step of engaging a proximal end ofthe distraction device with respect to the first vertebral body andengaging a distal end of the distraction device with respect to thesecond vertebral body. The distraction device may then be manipulated,to increase the distance between the first and second vertebral bodies.

The lumen may extend at least as far as the L4 vertebrae. The lumen maybe either linear, or curved. The forming step may comprise drilling.

The method may additionally comprise the step of removing at least apart of a disc from between the first and second vertebral bodies. Inone implementation of the method, the entire nucleus is removed frombetween the first and second vertebral bodies. Bone growth facilitatoror other media may thereafter be introduced through the lumen into thetreatment site. The distraction device may be left in place as animplant, or may be removed from the treatment site following thedistraction step.

In accordance with a further aspect of the present invention, there isprovided a method of treating the spine. The method comprises the stepsof identifying a site on the posterior surface of the sacrum, andforming a non-linear lumen from the site through the sacrum, through afirst vertebral body, and into a second vertebral body. A distractiondevice is thereafter introduced into the lumen.

In accordance with a further aspect of the present invention, there isprovided a method of increasing the distance between a first vertebralbody and a second vertebral body. The method comprises the steps ofproviding an internal distraction device, having an elongate body, afirst threaded surface on a proximal portion of the body and a secondthreaded surface on a distal portion of the body. The first threadedsurface is engaged with the first vertebral body, and the secondthreaded surface is engaged with the second vertebral body. Thedistraction device may thereafter be rotated, to increase the distancebetween the first vertebral body and the second vertebral body.

Further features and advantages of the present invention will becomeapparent to those of skill in the art in view of the detaileddescription of preferred embodiments which follows, when consideredtogether with the attached drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are lateral, posterior and anterior views of the lumbar andsacral portion of the spinal column depicting the visualized PAIFL andAAIFL extending cephalad and axially from the posterior laminectomy siteand the anterior target point, respectively;

FIG. 4 is a sagittal caudal view of a lumbar vertebrae depicting a TASIFaxial spinal implant or rod within a TASIF axial bore formed followingthe visualized PAIFL or AAIFL of FIGS. 1-3;

FIG. 5 is a sagittal caudal view of a lumbar vertebrae depicting aplurality, e.g., 2, TASIF axial spinal implants or rods within a likeplurality of TASIF axial bores formed in parallel with the visualizedPAIFL or AAIFL of FIGS. 1-3;

FIG. 6 is a simplified flow chart showing the principal surgicalpreparation steps of percutaneously accessing a posterior or anteriortarget point of the sacrum and forming a percutaneous tract followingthe visualized PAIFL or AAIFL of FIGS. 1-3, as well as subsequent stepsof forming the TASIF bore(s) for treatment of accessed vertebral bodiesand intervening discs and of implanting axial spinal implants therein;

FIG. 7 illustrates, in a partial cross-section side view, one manner ofobtaining access to a posterior target point for forming a posteriorTASIF axial bore through sacral and lumbar vertebrae and interveningdiscs axially aligned with the visualized PAIFL of FIGS. 1 and 2;

FIG. 8 is an enlarged partial cross section view illustrating aposterior TASIF axial bore through sacral and lumbar vertebrae andintervening discs axially aligned with the visualized PAIFL of FIGS. 1and 2;

FIG. 9 illustrates, in a partial cross-section side view, one manner ofobtaining access to an anterior target point for forming an anteriorTASIF axial bore through sacral and lumbar vertebrae and interveningdiscs axially aligned with the visualized AAIFL of FIGS. 1 and 2;

FIG. 10 is an enlarged partial cross-section view illustrating ananterior TASIF axial bore through sacral and lumbar vertebrae andintervening discs axially aligned with the visualized AAIFL of FIGS. 1and 2;

FIG. 11 illustrates, in a partial cross-section side view, one manner ofstrengthening the fusion of the vertebral bodies by implantation of anelongated axial spinal implant into the TASIF axial bore bridging thefilled or unaugmented disc space.

FIG. 11A illustrates, in side elevational, schematic view, anarticulating axial spinal implant in accordance with the presentinvention.

FIG. 12 illustrates, in a partial cross-section side view, one manner ofimplanting a spinal disc implant following discectomy of a disc effectedthrough the delivery of a porous envelope in a deflated conditionthrough a TASIF axial bore and into the disc space.

FIG. 13 illustrates the filling of the porous envelope of FIG. 12, witha bone growth material for fusion or other bio materials forming anartificial spinal disc implant.

FIG. 14 illustrates, in a partial cross-section side view, a combinationof above described therapies comprising the injection of bone growthmaterials or bone cement through an axial bore into cancellous bone oftwo vertebral bodies and intervertebral disc cavities before or after orsimultaneously with insertion of an elongated axial spinal implant intothe axial bore to effect a fusion with reinforcement of the vertebralbodies.

FIG. 15 is a schematic side elevational view of a dual lumenaugmentation media infusion catheter in accordance with one aspect ofthe present invention.

FIG. 16 is a cross sectional view taken along the line 16-16 of FIG. 15.

FIG. 17 is a cross sectional view taken along the line 17-17 of FIG. 15.

FIG. 18 is a side elevational schematic cross-section of an augmentationmedia infusion device injecting augmentation media into a vertebralbody.

FIG. 19 is a schematic side elevational cross-section as in FIG. 18,with the augmentation catheter proximally retracted and media infusedinto both the vertebral body and disc space.

FIG. 20 is a schematic side elevational view of a fusion implantpositioned axially across a disc space and into a cephalad and caudadvertebral body.

FIG. 21 is a schematic side elevational cross-sectional view of threefusion implants positioned across a disc space.

FIG. 22 is a side elevational view of a fusion implant, together withthe distal end of a deployment tool.

FIG. 23 illustrates, in a partial cross-section side view, a spinal discimplant having a planar spiral configuration that is delivered through aTASIF axial bore into the disc space and the retention of the spinaldisc implant by implantation of an elongated axial spinal implant intothe TASIF axial bore bridging the filled disc space.

FIG. 24 is an axial view, taken along lines 24-24 of FIG. 23illustrating the spiral configuration of the spinal disc implant.

FIG. 25 is a side view of an elongated axial spinal implant providing adistraction function.

FIG. 26 is a side view of an elongated axial spinal implant providing ashock absorbing function.

FIG. 27 illustrates, in a partial cross-section side view, the insertionof an axial spinal implant providing distraction or shock absorbing oftwo or more vertebrae across one or more intervertebral disc into aTASIF axial bore.

FIG. 28 illustrates, in side elevational, schematic view, one embodimentof an implantable spinal distraction/fusion rod with varied thread pitchand diameter along its length.

FIG. 29A is a side, schematic view of one embodiment of adistraction/fusion rod.

FIG. 29B is a side, schematic view of one embodiment of adistraction/fusion rod illustrating the major and minor diameters of thescrew threads.

FIG. 30 is a cross-section side view of one embodiment of adistraction/fusion rod having a root portion that is hollow toward thetrailing end.

FIG. 31 is a cross-section side view of one embodiment of adistraction/fusion rod having a root portion that is hollow through outits entire length.

FIG. 32 illustrates, in side elevational, schematic view, one embodimentof an implantable spinal distraction/fusion rod with a plurality ofapertures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The methods and surgical instrumentation and axial spinal implantsdisclosed in co-pending, commonly assigned, related patent applicationsU.S. patent application Ser. No. 09/640,222 filed Aug. 16, 2000, U.S.patent application Ser. No. 09/684,820 filed Oct. 10, 2000, U.S. patentapplication Ser. No. 09/710,369 filed Nov. 10, 2000, U.S. patentapplication Ser. No. 09/709,105 filed Nov. 10, 2000, U.S. patentapplication Ser. No. 09/782,583 filed Feb. 13, 2001, U.S. patentapplication Ser. No. 09/848,556 filed May 3, 2001, U.S. patentapplication Ser. No. 10/125,771 filed Apr. 18, 2002, U.S. patentapplication Ser. No. 09/782,534 filed Feb. 13, 2001 can be employed inthe practice of the present invention, and the disclosures of theabove-identified applications are hereby incorporated in theirentireties herein by reference.

FIGS. 1-3 schematically illustrate the anterior and posterior TASIFsurgical approaches in relation to the lumbar region of the spinalcolumn, and FIGS. 4-5 illustrate the location of the TASIF implant orpair of TASIF implants within a corresponding posterior TASIF axial bore22 or anterior TASIF axial bore 152 or pair of TASIF axial bores 22 ₁,22 ₂ or 152 ₁, 152 ₂. Two TASIF axial bores and axial spinal implants orrods are shown in FIG. 5 to illustrate that a plurality, that is two orthree or more, of the same may be formed and/or employed in side by siderelation in parallel alignment with the AAIFL or PAIFL or diverging fromthe AAIFL or PAIFL in the cephalad direction. Preferred TASIF surgicalapproaches for providing anterior and posterior trans-sacral accessdepicted in FIGS. 1-3 and preparing the TASIF axial bores 22 or 152 or22 ₁, 22 ₂, or 152 ₁, 152 ₂ shown in FIGS. 4 and 5 are illustrated inthe above-referenced '105 and '748 applications.

The lower regions of the spinal column comprising the coccyx, fusedsacral vertebrae S1-S5 forming the sacrum, and the lumbar vertebraeL1-L5 described above are depicted in a lateral view in FIG. 1. Theseries of adjacent vertebrae located within the human lumbar and sacralspine have an anterior aspect, a posterior aspect and an axial aspect,and the lumbar vertebrae are separated by intact or damagedintervertebral spinal discs labeled D1-D5 in FIG. 1. FIGS. 2 and 3depict the posterior and anterior views of the sacrum and coccyx.

The method and apparatus for forming an anterior or posterior TASIFaxial bore initially involves accessing an anterior sacral position,e.g. an anterior target point at the junction of S1 and S2 depicted inFIGS. 1 and 3, or a posterior sacral position, e.g. a posteriorlaminectomy site of S2 depicted in FIGS. 1 and 2. One (or more)visualized, imaginary, axial instrumentation/fusion line extendscephalad and axially in the axial aspect through the series of adjacentvertebral bodies, L4 and L5 in this illustrated example. The visualizedAAIFL through L4, D4, L5 and D5 extends relatively straight from theanterior target point along S1 depicted in FIGS. 1 and 3, but may becurved as to follow the curvature of the spinal column in the cephaladdirection. The visualized PAIFL extends in the cephalad direction withmore pronounced curvature from the posterior laminectomy site of S2depicted in FIGS. 1 and 2. A preoperative CT scan or magnetic resonanceimaging (MRI) study of the patient's spine is conducted to visualize andmap the AAIFL or PAIFL.

FIG. 6 depicts, in general terms, the surgical steps of accessing theanterior or posterior sacral positions illustrated in FIGS. 1-3 (S100)forming posterior and anterior TASIF axial bores (S200), optionallyinspecting the discs and vertebral bodies, performing discectomy ordiscoscopy, disc augmentation, and vertebral bone reinforcement,balloon-assisted vertebroplasty or vertebroplasty (S300), and implantingposterior and anterior axial spinal implants and rods or plugs into theaxial bore(s) (S400) in a simplified manner. In step S100, access to theanterior or posterior sacral position, that is the anterior target pointof FIG. 3 or the posterior laminectomy site of FIG. 2 is obtained, andthe anterior or posterior sacral position is penetrated to provide astarting point for each axial bore that is to be created. Then, one ormore axial bore is bored from each point of penetration extending inalignment with either the PAIFL or AAIFL cephalad and axially throughthe vertebral bodies of the series of adjacent vertebrae and anyintervertebral spinal discs (S200). The axial bore(s) can traverse oneor more vertebral body cephalad to the sacral vertebral bodies S1, S2and any intervertebral disc and can terminate at a cephalad end within aparticular vertebral body or spinal disc. The axial bore may be visuallyinspected using an endoscope to determine if the procedures of step S300should be performed.

The performance of step S100 in the anterior and/or posterior TASIFprocedures may involve drilling a pilot hole, smaller in diameter thanthe TASIF axial bore, in the prescribed alignment with the AAIFL and/orPAIFL in order to complete the formation of the anterior and/orposterior percutaneous tracts. Certain of the therapeutic procedures ofsteps S300 and S400 may optionally be completed through the AAIFL/PAIFLpilot hole following step S100, rather than following the enlargement ofthe pilot hole to form the TASIF axial bore in step S200.

Step S100 preferably involves creation of an anterior or posteriorpercutaneous pathway that enables introduction of further tools andinstruments for forming an anterior or posterior percutaneous tractextending from the skin incision to the respective anterior or posteriortarget point of the sacral surface or, in some embodiments, the cephaladend of a pilot hole over which or through which further instruments areintroduced as described in the above-referenced '222 application. An“anterior, presacral, percutaneous tract” 26 (FIG. 1) extends throughthe “presacral space” anterior to the sacrum. The posterior percutaneoustract or the anterior, presacral, percutaneous tract is preferably usedto bore one or more respective posterior or anterior TASIF bore in thecephalad direction through one or more lumbar vertebral bodies andintervening discs, if present. “Percutaneous” in this context simplymeans through the skin and to the posterior or anterior target point, asin transcutaneous or transdermal, without implying any particularprocedure from other medical arts. However, percutaneous is distinctfrom a surgical access, and the percutaneous opening in the skin ispreferably minimized so that it is less than 4 cm across, preferablyless than 2 cm, and, in certain applications, less than 1 cm across. Thepercutaneous pathway is generally axially aligned with the AAIFL or thePAIFL extending from the respective anterior or posterior target pointthrough at least one sacral vertebral body and one or more lumbarvertebral body in the cephalad direction as visualized by radiographicor fluoroscopic equipment.

It should be noted that the formation of the anterior tract 26 shown inFIG. 1 through presacral space under visualization described above isclinically feasible as evidenced by clinical techniques described by J.J. Trambert, M D, in “Percutaneous Interventions in the Presacral Space:CT-guided Precoccygeal Approach-Early Experience (Radiology 1999;213:901-904).

Certain of the therapeutic procedures of the present invention areconducted through relatively straight or curved anterior TASIF bores orcurved posterior TASIF bores or pilot holes. Introduction of axialspinal implants and instruments for performing discoscopy to inspect theaccessed discs, discectomies and/or disc augmentation/replacement and/orvertebroplasty, balloon-assisted vertebroplasty, fusion, alignment, drugdelivery, electrical stimulation, or other therapies, is enabled by theprovision of the percutaneous pathway and formation of the anterior orposterior TASIF bore(s).

The bore forming tool sets comprise elongated drill shaft assembliessupporting distal boring tools, e.g., mechanical rotating drill bits,burrs, augurs, abraders, or the like (collectively referred to as boringheads or drill bits for convenience), that can be manipulated in use tobore a straight or curved axial bore. Suitable bore forming tools aredisclosed in the above-referenced United States Patent Applications.However, the TASIF axial bores can be formed by other tools thatmechanically puncture or penetrate vertebral bodies and intervertebraldiscs or otherwise form TASIF axial bores in any diameter orcross-section and that follow any alignment with the axis of the spineas visualized by the AAIFL or PAIFL. For convenience, the posterior andanterior TASIF axial bores are referred to as being formed or boredherein.

FIGS. 7 and 8 illustrate step S100 for forming the posteriorpercutaneous tract and the posterior TASIF axial bore 22 formed in stepS200 and extending through sacral and lumbar vertebrae and interveningdiscs axially aligned with the visualized PAIFL of FIGS. 1 and 2 using aboring tool of the type described in more detail in the above-referenced'105 application. The same steps can be employed to form a pilot hole ofstep S100 that can be enlarged in step S200. In this case, a smalldiameter bore forming tool (e.g. 3.0 mm diameter) is used to first borea small diameter curved pilot hole following the imaginary, visualizedPAIFL 20, such as through S1, L5 and L4 in step S100. Then, the boringtool is removed, and a guidewire having a threaded distal screw-in tipis advanced through the pilot hole and screwed into to the caudal end ofthe pilot hole and into cephalad portion of the L4 body. Anover-the-wire bore enlarging tool having a flexible body capable oftracking the curved guidewire is fitted over the proximal end of theguidewire and manually or mechanically rotated and advanced along it instep S200. In this way, the small pilot hole diameter is enlarged toform the anterior TASIF axial bore 22 having a diameter e.g. a 10.0 mmdiameter, and the enlarging tool is then removed.

It will be understood that the illustrated diameter of the posteriorTASIF axial bore hole 22 relative to sizes of the vertebral bodies ismerely exemplary, and that it is contemplated that the pilot hole andbore hole diameters can range from about 1-10 mm and 3-30 mm,respectively. Moreover, it will be understood that a plurality of suchposterior TASIF axial bores 22 ₁, . . . 22 _(n) can be formed in side byside or diverging relation generally aligned with the PAIFL. Althoughmore can be used and remain within the scope of the present invention,one or two or three or four side by side (parallel or diverging) axialbores are presently contemplated.

In FIG. 7, the posterior surface of the sacrum is exposed in step S100as described in the above-referenced applications. The area of thepatient's skin surrounding the incision site is surgically prepped, andthe anus is excluded from the surgical field using adhesive drapes. Theactual dermal entry site may be determined by the prone, preoperative CTscan or magnetic resonance imaging (MRI) study that maps the PAIFL. Instep S100, an incision is made in the patient's skin over the posteriorsacral surface of S2, and the subcutaneous tissue is separated to exposethe posteriorly extending, bony ridge of the posterior sacral surface. Asmall laminectomy 14 is performed through the posterior ridge of thesacrum inferior. The thecal sac and nerve roots that are exposed by thelaminectomy are gently retracted, and the terminal portion of the spinalcanal is exposed.

An elongated drill shaft assembly (not shown) is axially aligned withthe PAIFL at the posterior target point so that the initial penetrationof the sacrum is substantially at right angles to the exposed sacralsurface. A drill guide for receiving the drill drive shaft assembly fordrilling or boring a posterior TASIF axial bore 22 from S2 along thevisualized PAIFL may optionally be attached to S2 and extendedposteriorly through the exposed spinal canal and skin incision.

The progress of the drill bit is observed using conventional imagingequipment. As the elongated drill shaft assembly is extended anteriorlyin the cephalad direction, a curvature is introduced in the cephaladsegment of the posterior TASIF axial bore 22 as shown in FIG. 8. It isnecessary to maintain the plane of curvature of the distal segmentaligned to the curvature of the spine. In this way, the drill bitadvances through the sacral vertebrae in the cephalad direction andtoward the lumbar vertebral bodies while staying within the spongy,cancellous bone of each vertebral body. Theoretically, any number ofvertebral bodies of the spine can be bored through in the cephalad axialdirection. The cephalad end of the posterior TASIF axial bore 22 canterminate within a vertebral body or within a disc or disc space.

FIGS. 9 and 10 illustrate the anterior percutaneous tract formed in stepS100 and the anterior TASIF axial bore 22 formed in step S200 andextending through sacral and lumbar vertebrae and intervening discsaxially aligned with the visualized AAIFL of FIGS. 1 and 2 using aboring tool of the type described in more detail in the above-referencedapplications. The same steps can be employed to form a pilot hole ofstep S100 that can be enlarged in step S200 as described above. It willbe understood that the illustrated diameter of the anterior TASIF axialbore hole 152 relative to sizes of the vertebral bodies is merelyexemplary, and that it is contemplated that the pilot holes and borehole diameters can range from about 1-10 mm and 3-30 mm, respectively.Moreover, it will be understood that a plurality of such anterior TASIFaxial bores 152 ₁ . . . 152 _(n) can be formed in side by side ordiverging relation generally aligned with the AAIFL.

The anterior TASIF axial bore(s) can be relatively straight from theanterior target point into or through at least the caudal lumbarvertebrae and intervertebral discs. But, it may be desirable ornecessary to form a curved anterior TASIF axial bore(s) particularly asthe bore(s) is extended in the cephalad direction to maintain the planeof curvature of the cephalad segment of the TASIF axial bore(s) alignedto the curvature of the spine. In this way, the drill bit advancesthrough the sacral vertebrae in the cephalad direction while stayingwithin the spongy, cancellous bone of each vertebral body.Theoretically, any number of vertebral bodies of the spine can be boredthrough in the cephalad direction. The cephalad end of the posteriorTASIF axial bore(s) 152 can terminate within a vertebral body or withina disc or disc space.

In accordance with the present invention, a variety of therapeuticprocedures can be performed in step S300 or step S400 after the curvedposterior or curved or straight anterior TASIF axial bore(s) is formedemploying instruments, axial spinal implants, spinal disc implants andmaterials. Certain of the therapies or therapeutic procedures can becompleted in step S300 without an axial spinal implant also beingimplanted in step S400.

Step S400 may also be performed in certain cases without a therapeuticprocedure being performed in step S300. The therapeutic procedures thatare performed using such instruments, axial spinal implants, spinal discimplants and materials of the present invention include, but are notlimited to one or more of: (1) performing a discoscopy by inserting anendoscope and inspecting the condition of the vertebral bodies andspinal discs; (2) performing a simple fusion by inserting bone growthmaterials into the TASIF axial bore(s) in step S400; (3) performing apartial discectomy or complete discectomy of a disc accessed through aTASIF axial bore in step S300; (4) performing a vertebroplasty orballoon-assisted vertebroplasty to a fractured vertebral body accessedthrough a TASIF axial bore in step S300; (5) inserting an artificialdisc implant or autologous/homologous bone or bone growth material intothe disc space following a complete discectomy of a disc accessedthrough a TASIF axial bore in step S400 to encourage fusion or tofunction as a functional disc replacement; (6) following a partialdiscectomy removing at least a portion of the nucleus, inserting aninflatable envelope or other disc implant or a material into the discspace to augment a disc accessed through a TASIF axial bore in step S300to encourage fusion or to function as a functional disc replacement; (7)inserting axial spinal implants into the bore(s) as a single therapy orin conjunction with any of the preceding listed therapies (1)-(6) instep S400; (8) inserting axial spinal implants providing distraction oftwo or more vertebrae across one or more intervertebral disc and/orshock absorption due to loads applied axially to the spine in step S400;(9) extending an electrical lead from an implanted or externalelectrical stimulator through a TASIF axial bore to locate one or moreelectrical stimulation electrode of the lead or incorporated with anelongated axial spinal implant or spinal disc implant within or betweenadjoining vertebral bodies to apply electrical stimulation to encouragebone growth or to counter pain in step S300; (10) extending a catheterfrom an implanted or external drug dispenser through a TASIF axial boreto a drug delivery port of the catheter or incorporated with anelongated axial spinal implant or spinal disc implant to dispense a drugwithin or between adjoining vertebral bodies or outside vertebral bodiesto encourage bone growth or to counter pain in step S300; and (11)performing a brachytherapy of a vertebral body through an axial bore totreat metastatic disease in the spine or adenopathy in theretroperitoneum. The TASIF axial bore openings at the anterior orposterior sacral positions are preferably backfilled, plugged or closedfollowing each such therapeutic procedure with a bone growth material,bone cement and/or prosthetic plug or cap.

The axial insertion of spinal implants referenced above includes theimplantation of any of a wide variety of implants, some of which aredescribed elsewhere herein. For example, implants in the form of a solidrod of either polymeric or metal construction may be utilized.Modifications to the solid rod include surface texturing, interferencefit structures for engaging cancellous or cortical bone, and the like.The rods may additionally include a drug delivery capability, such thatany of a variety of drugs may elute from the implant, or be released asthe implant degrades in the event of a bioabsorbable implant. Eithermetal or polymeric rods may be provided with a variety of apertures, forfacilitating bone ingrowth as is understood in the art. For example, avariety of structures are known in the art and presently implantedtransversely to the axis of the spine, known as cages. A wide variety ofcages may be utilized with or without modification in the context of thepresent invention by axial advance into the axially extending boresdescribed herein. Such cages often include one or more central lumen, incommunication with the exterior through the side wall of the cage by wayof a plurality of side wall apertures. The external surface of the sidewall may be provided with a helical thread, or other radially outwardlyextending structure for providing mechanical interference fit with theadjacent bone.

In addition, any of the foregoing procedures may be accompanied by avisualization step. Following creation of the axial bore, avisualization device such as an endoscope may be transluminally advancedthrough the bore to permit inspection of the treatment zone.Visualization, whether endoscopic, fluoroscopic, etc., allows forobservation of the health and general condition of the vertebral bodiesand intervertebral discs, as well as assessment of the progress orresult of a nucleus removal or full disc removal procedure. In addition,therapeutic and/or diagnostic procedures may be monitored under directvisualization through the axial bores created in accordance with thepresent invention.

For convenience of illustration, the therapeutic procedures areillustrated in the drawings and described as follows as being performedthrough an anterior percutaneous tract formed using an anterior tractsheath 96 and TASIF axial bore 152. But, it will be understood that theprocedures may be performed through a posterior percutaneous tract andTASIF axial bore 22 and that certain of the procedures may beadvantageously performed using two or more parallel or diverging TASIFaxial bores.

In each of the following procedures to deliver a therapy, the anteriorTASIF axial bore 152 is formed, as described above, through the use ofanterior tract sheath 96 (FIG. 9) inserted earlier through the presacralspace 24 from a skin incision 28 to the anterior target point of theanterior surface of sacral vertebra S1 that defines the percutaneoustract 26. The shaped end 98 of the anterior tract sheath 96 is alignedwith the anterior surface of the sacral vertebra S1 during step S100.The shaped end 98 may be formed with attachment teeth or threads to fixit to the sacral bone. It will be understood that the therapeuticprocedures of the present invention may be performed through the lumenof such a tract sheath 96 or simply through a defined anterior tract 26extending through the pre-sacral space 24 to axially access thevertebrae.

It will be understood that each of the following therapies to the spinaldiscs or the vertebral bodies can be conducted on more than one spinaldisc or vertebral body or on one or more spinal disc and one or morevertebral body traversed by at least one TASIF axial bore. For example,two, or three or four or five or more spinal discs may be accessed by asingle TASIF axial bore, and treated in one of the following ways,generally starting with the cephalad spinal disc. Then, the portion ofthe TASIF axial bore between the cephalad and caudal spinal disc may beclosed by an artificial axial spinal implant or bone growth material asappropriate. The caudal spinal disc is then treated, and the portion ofthe TASIF axial bore between the caudal spinal disc and the anterior orposterior sacral bore entry point may then be closed by an artificialaxial spinal implant or bone growth material as appropriate. Similarly,cephalad and caudal vertebral bodies may be treated by vertebroplasty orballoon-assisted vertebroplasty, and the intervertebral disc may also betreated by one of the following described therapies. For convenience,the treatment of only a single spinal disc or vertebral body isdescribed and illustrated in the drawings.

Thus, the following exemplary therapeutic procedures of the presentinvention are understood to involve accessing an anterior or posteriorsacral position of a sacral vertebra in alignment with the visualized,AAIFL or PAIFL extending in said axial aspect cephalad through a seriesof adjacent vertebral bodies of adjacent vertebrae. Then, from theaccessed anterior sacral position, at least one anterior or posteriorTASIF axial bore is bored in alignment (as defined herein) with theAAIFL or PAIFL axially through at least the caudal sacral vertebra andthrough or into one or more cephalad vertebral bodies of the series ofadjacent vertebral bodies and any interposed, intervertebral, spinaldiscs. The delivery of the therapies is followed by the withdrawal ofany tract forming tools and a simple surgical closure of the incisionsite. The therapies can be delivered or therapeutic procedures can beperformed as follows. The procedures below are merely representative ofthose contemplated using the access methods of the present invention.

One simple fusion therapeutic procedure of the present inventioninvolves filling the anterior or posterior TASIF axial bore(s) with abone growth material which bridges the spinal disc and will effect bonegrowth across the spinal disc. The cancellous bone is typically porouswith fissures and cavities, so that the bone growth material is alsoforced into such cancellous bone cavities and fissures. In thisapplication, it may be desirable to bore and fill a plurality ofparallel or diverging anterior or posterior TASIF axial bores to providea number of bridges of bone growth material through the intervertebralspinal disc.

For purposes of this therapy and other fusion therapies describedherein, a “bone growth material” can be one or more of the following, orany other biocompatible material judged to have the desired physiologicresponse, including any natural or artificial osteoconductive,osteoinductive, osteogenic, or other fusion encouraging material.Particularly, morselized cortical, cancellous, or cortico-cancellousbone graft, including autograft, allograft, or xenograft might beemployed. Or any bone graft substitute or combination of bone graftsubstitutes, or combinations of bone graft and bone graft substitutes,or bone inducing substances, could be employed. Such bone graftsubstitutes or bone inducing substances include, but not limited to,hydroxyapatite, hydroxyapatite tricalcium phosphate; bone morphogenicprotein (BMP) and calcified or decalcified bone derivative andresorbable bone cements. The resorbable cement material can be a calciumderivative generally composed of hydroxyapatite, orthophosphoric acid,calcium carbonate, and calcium hydroxide formed into a semi-liquid pastewith an alkaline solution of sodium hydroxide and water or a compositioncomprising polypropylene fumarate or a mixture of calcium phosphates.Other compositions that may be employed comprise calcium salt filler,N-vinyl-2-pyrrolidone, and a peroxide or free radical initiator. Thebone graft material may be mixed with a radiographic material to enableits visualization during delivery to assure proper disposition andfilling of bores, cavities and spaces described herein.

In certain cases, e.g. correcting spondylolisthesis, it is necessary torealign the vertebral bodies before boring the anterior or posteriorTASIF axial bore(s) and to reinforce or stabilize the vertebrae pendingfusion. In this case, and in other cases where reinforcement is deemednecessary, a pre-formed elongated axial spinal implant can be insertedinto at least one of the anterior or posterior TASIF axial bore(s) alongwith bone growth material. The axial spinal implant can be a surfaceroughened metal rod or porous tube that is configured to the particularbore curvature and size and surface treated to bite into vertebral boneand/or to promote bone ingrowth.

This therapy provides a simple and relatively atraumatic approach to thevertebrae of interest where there is no need to treat or remove theintervertebral disc. Fusion may be effected in other ways described asfollows using the bone growth materials and optionally using anelongated axial spinal implant.

As described above, the complete and partial discectomy proceduresfollowed by introduction of fusion materials and devices conducted inthe past have been done through lateral exposure of the disc thatpresents a number of problems that are eliminated by the presentinvention.

In its simplest form of this aspect of the present invention, fusion ofthe prepared endplates of, for example, L4 and L5 can be effected bypumping a bone growth media into the disc space 154 through the TASIFaxial bore 152 and percutaneous tract 26. A catheter having a straightor flexible tip section can be employed that is extended through theaxially aligned anterior tract 26 and the TASIF axial bore 152 todispense the bone growth material in the disc space 154. A plunger canalso be employed that is extended through the axially aligned anteriortract 26 and the TASIF axial bore 152 to pack the dispensed materialfrom the TASIF axial bore 152 into the disc space 154. Then, the caudalend opening or the full length of the TASIF axial bore 152 is pluggedwith an artificial plug or bone growth material, and the sheath 96 iswithdrawn. The incision 28 is closed and the patient rests for a timeuntil bone growth takes place.

This procedure may suffer from the inability to adequately fill the discspace 154 or to confine the dispensed bone growth material within thedisc space during the procedure and may require prolonged bed restrecuperation to avoid later settling and ejection of the bone growthmaterial.

A pre-formed, rod shaped, artificial axial spinal implant 70 can beinserted into the TASIF axial bore 152 to fill all or part of it inorder to help maintain the distraction of the vertebral bodies as shownin FIG. 11. The pre-formed axial spinal implant 70 can extend into thecephalad vertebral body L4 if the TASIF axial bore 154 extends into itas shown in FIG. 11 and may include the distraction and shock absorbingcharacteristics of the particular axial spinal implants described below.In this case, it may be desirable to form parallel or diverging TASIFaxial bores and implant an axial spinal implant in each bore. Thepre-formed, rod shaped, artificial axial spinal implant 70 may employ afixation mechanism and/or be fixed in place using one of the abovedescribed bone growth materials.

The spinal implant 70 can take any of a variety of forms as will beappreciated by those of skill in the art in view of the disclosureherein. For example, surface structures may be provided for enhancingbone ingrowth for resistance to axial compression or elongationfollowing a bone ingrowth period. Any one or combination of structuressuch as surface texturing, helical threads, radially outwardly extendingflanges or arms may be utilized to enhance the integrity of thejunction. In addition, bone ingrowth may be facilitated by providing thespinal implant 70 with a central lumen extending at least part way andpreferably throughout the axial length of the device. A plurality ofside wall openings may also be provided, to enable communication betweenthe adjacent bone and the interior of the spinal implant 70. A varietyof structures are presently known which meet this general description,and examples may be found in U.S. Pat. No. 6,287,343 to Kuslich et al.and 4,501,269 to Bagby, the disclosures of which are incorporated intheir entireties herein by reference. The tubular implant havingperforated side walls may additionally be provided with an externalhelical thread, to further enhance the integrity of the resulting joint.A single spinal implant 70 or multiple spinal implants may be implantedwithin a treatment zone in the spine, as is discussed elsewhere herein.

Referring to FIG. 11 a, there is illustrated an articulating spinalimplant 71 in accordance with another aspect of the present invention.The articulating implant may be used to maintain the spacing betweenvertebral bodies, and/or maintain a desired spinal alignment, whilepreserving flexibility at the discs.

The articulating spinal implant may be transluminally positioned withinthe TASIF axial bore 152, to span two or three or four or five or moreadjacent vertebral bodies. Articulating spinal implant 71 comprisesgenerally a proximal end 73, a distal end 75 and an elongate articulatedbody extending therebetween. The body comprises a plurality of segments77, (e.g. two or three or four or more) each separated by anarticulation or flexible joint 79. In the implanted orientation, eachsegment 77 axially corresponds to a vertebral body, and each joint 79corresponds to a disc. Thus, the axial length of each segment 77approximates the column height of the intended corresponding vertebralbody, and the axial length of each joint 79 approximates the columnheight of the corresponding disc.

Each segment 77 may additionally be provided with one or more anchoringand/or ingrowth surface structures to facilitate cancellous bone and/orcortical bone ingrowth, and to anchor the segment 77 within thevertebral body. In the illustrated embodiment, each segment 77 isprovided with a plurality of apertures 81 to permit cancellous boneingrowth and/or permit the expression of one or more bone growthmaterials. For this latter application, at least some and preferably allof the apertures 81 are in communication with a central lumen (notillustrated) which is accessible at proximal end 73 by an infusiondevice positioned outside of the body. One or more suitable bone growthmaterials, such as those discussed above, are expressed through thelumen, and out of the apertures 81 as will be understood by those ofskill in the art in view of the disclosure herein. In addition to, orinstead of the apertures 81, any of a wide variety of surface structuressuch as pitted or textured surfaces, radially outwardly extendinganchors, recesses or projections may be included, to facilitateanchoring within the bone following a post procedure healing period.

The joints 79 may be formed in any of a variety of manners, such as bythe provision of a bellows shaped plurality of alternating annularrecesses and ridges. The material of the adjacent segments 77 may bereduced in diameter at the joints 79 to increase lateral flexibility ata living hinge. Alternatively, a ball joint or mechanical pivot such astwo arms or sidewalls connected by and rotatable about a connector suchas a hinge pin, rivet or the like may be used. A metal coil spring orbiocompatible compressible elastomeric materials may also be used topermit limited flex and axial compressibility depending upon the desiredpost implantation performance.

The articulating implant 71 may be formed in any of a variety of ways,which will be apparent to those of skill in the art. For example, all orportions of the implant 71 may be formed by molding or extrusion of anyof a variety of polymeric materials well known in the medical devicearts, such as PEEK, PET, nylon, PTFE, various densities of polyethylene,and the like. Alternatively, any of a variety of solid or tubular metalstock such as stainless steel, titanium or nitinol may be utilized.Hinge point 79 may be machined in, molded in or fabricated duringsubsequent assembly processes depending upon the hinge design. Theoverall dimensions of the articulating implant 71 will be selecteddependent upon the intended use environment. In general, for an adulthuman patient, the outside diameter of the implant 71 will be within therange of from about 0.25 inches to about 1 inch. The length of theimplant 71 will depend upon the number of discs and vertebral bodieswhich the implant 71 is intended to span.

Alternatively, fusion of the vertebral bodies can be effected accordingto a further aspect of the present invention using spinal disc implantsthat are dispensed into the disc space through the TASIF axial bore 152and percutaneous tract 26 and maintained there in a variety of ways. Oneapproach, shown in FIG. 12, is to dispense a porous, deflated, shapedbag or balloon or sack or other envelope 80 of a type described in theabove-referenced '679 patent into the disc space 154 and to then filland inflate the envelope 80 with a bone growth material 60 of one of thetypes described above as shown in FIG. 13, and to close the opening intothe envelope 80. The porous fabric has pores that are small enough toconfine the bone growth material within the envelope while allowingpassage of fluids and bone growth therethrough. The envelope 80 could beformed of a tightly woven, high molecular weight, high tenacity,flexible polymeric fabric e.g., high molecular weight polyethylene,polyester, polyolefine, polyethylene terephthalate,polytetrafluoroethylene, polysulfone, nylons, or any other highmolecular weight, and other high tenacity materials including carbonfiber yarns, ceramic fibers, metallic fibers, etc.

The envelope 80 can be inserted into the prepared disc space 154 in avariety of ways, one of which is shown in FIG. 12. The envelope 80 isfolded against a blunt tip flexible push wire 52 which extends throughthe lumen of a fill tube 54 that extends into the balloon opening. Thefolded envelope 80, push wire 52 and fill tube 54 are in turn insertedinto the lumen of a tubular catheter or sheath 56 that is advancedthrough the percutaneous tract 26 and TASIF axial bore 152. Then, theenvelope 80 is advanced into the prepared, distracted disc space 154 andspread out by pushing and torqueing the push wire 52. Air or liquidinflation of the shaped envelope 80 can also be used to spread theenvelope out in the disc space instead of the push wire 52.

The push wire 52 is withdrawn, air is evacuated from the envelopeinterior through the lumen of the fill tube 54, and bone growth material60 is then injected through the lumen of the fill tube 54 as shown inFIG. 13. The filled envelope 80 conforms with the prepared disc space154, and the dispensed bone growth material 60 is confined therein tomaintain the spacing between the vertebral body end plates when theenvelope opening is closed. The envelope opening can be closed bydrawing a purse-string suture sewn around the opening tight and tying itthrough the TASIF axial bore 152 and percutaneous tract 26 or in othermanners described in the above-referenced '679 and '736 patents and inU.S. Pat. No. 5,702,454. The bone growth material confined within theenvelope 80 maintains the distraction spacing between the preparedvertebral body endplates.

One manner of maintaining the filled envelope 80 in place would be toadvance an elongated axial spinal implant 70 in the caudal directioninto the opening of the filled envelope 80 to abut a section of theenvelope fabric against the cephalad vertebral endplate, thereby bothsealing the envelope opening and locking it into place.

Another way to maintain the filled envelope 80 in place would be to formthe disc space 154 with concave, prepared vertebral body endplatesduring the complete discectomy and to form the envelope 80 with convexsides that fit within the concave surfaces when the envelope 80 isfilled with bone growth material.

As described above, the complete and partial discectomy proceduresfollowed by a functional disc replacement implant conducted in the pasthave been done through lateral exposure of the disc that presents anumber of problems that are eliminated by the present invention.

In this aspect of the present invention, various forms of spinal discimplants that mimic the function and characteristics of a natural discwithout promoting fusion of the cephalad and caudal vertebral bodies canbe inserted into the disc space through the TASIF axial bore 152 andpercutaneous tract 26 and maintained there in a variety of ways. Oneapproach is to dispense a deflated, shaped bag or balloon or sack orother envelope of a type described in the above-referenced '326 and '454patents and in U.S. Pat. Nos. 5,888,220 and 5,562,736 into the discspace 154. Then, the envelope 80 is filled and inflated with a curablebiomaterial that does not necessarily encourage bone growth andpreferably is resilient. The '326, '220 and '736 patents describeperforming a discectomy through lateral approaches to remove the nucleuswhile retaining as much of the annulus as possible, given the lateralpenetration through it. In accordance with this aspect of the presentinvention, the annulus is removed in the complete discectomy describedabove, and the deflated envelope 80 is inserted and inflated through theTASIF axial bore 152 and percutaneous tract 26 as shown in FIG. 12 anddescribed above. The envelope 80 conforms with the prepared disc space,and the dispensed biomaterial is confined therein to maintain thespacing between the vertebral body end plates when the envelope openingis closed. The dispensed biomaterial confined within the envelopemaintains the distraction spacing between the prepared vertebral bodyendplates but does not necessarily encourage fusion.

In this case, the biomaterial filling the envelope 80 is a material thatis capable of being introduced to the site of a joint by minimallyinvasive means, and be hydrated or cured in place to provide desiredphysical-chemical properties as described, for example, in theabove-referenced '326, '454, '220 and '736 patents. A hydrogel can beinjected into the envelope 80 in a liquid or dry particulate form or inmicrospheres or beads in the manner shown in FIG. 13. A preferredhydrogel is formulated as a mixture of hydrogel polyacrylonitrile or anyhydrophilic acrylate derivative with a unique multiblock copolymerstructure or any other hydrogel material having the ability to imbibeand expel fluids while maintaining its structure under various stresses.For example, the hydrogel can be formulated as a mixture of polyvinylalcohol and water. The hydrogel core formed within the envelope 80 willswell as it absorbs fluids through the porous fabric wall of theenvelope 80, preferably in the manner of a native nucleus. When fullyhydrated, the hydrogel core may have a water content of between 25-95%.The hydrogel material of one suitable embodiment is manufactured underthe trade name Hypan® by Hymedix International, Inc., and has a watercontent of about 80-95%. In addition, any of the hydrogels and solventsidentified in the above-referenced '326 patent may be employed to fillthe envelope 80.

The preferred woven construction of the envelope creates a plurality ofsmall openings large enough to allow bodily fluids to interact with thehydrogel core, but small enough to prevent the hydrogel from escaping.Preferably, the openings have an average diameter of approximately 10micrometers, although other dimensions are acceptable. While the fabricis described as woven, any other configuration having a semi-permeableor porous attribute can be used. The flexible material allows expansionand contraction of the hydrogel core in a controlled fashion. Thehydrogel core acts as a cushion against various loads placed upon it. Tohelp achieve this effect, the preferred envelope fabric is flexible andsemi-elastic, having a burst strength that is greater than the swellingpressure of the hydrogel core when fully hydrated to prevent rending andloss of the hydrogel core. By having an envelope or jacket that is bothflexible and semi-elastic, with the semi-elasticity being “modulusmatched” to the native nucleus, the filled envelope or jacket acts as ashock absorber and properly assumes and distributes loads. This incontrast to the flexible and inelastic jacket disclosed in the '736patent. When fully hydrated, an inelastic jacket of the type disclosedin the '736 patent may have expanded to its full capacity. As such, itmay not stretch or give when a load is applied and cannot deform sincethe compression modulus in the normal load range is nearly vertical orincompressible.

As mentioned previously, a suitable membrane or envelope will preferablybe both flexible and semi-elastic to enable conformability, flexibility,and controlled load sharing. The load sharing is “modulus-matched” tothat of the natural nucleus, vertebral endplates, and the annulus. Themodulus in question is actually both bulk modulus as well as compressionmodulus. For example, in one embodiment, the membrane can be tightlywoven or knit from polymeric fibers, such as, for example, Dacron. Inanother embodiment, the membrane can be configured of such fibers usingan available non-woven technology, such as that used in the manufactureof Tyvek™ film. In yet another embodiment, the membrane can beconfigured from polymeric film that has been produced or modified tobecome microporous. For example, a biaxially oriented balloon is anexample of a polymeric film membrane. Microporosity can be achieved innumerous ways, such as, for example, mechanically by means of laserdrilling or chemically by incorporating sacrificial particles such as asalt that can be leached out after the final configuration is achieved.

This therapeutic procedure of the present invention provides a shockabsorbing functional disc replacement of the nucleus of the spinal discand can be advantageously conducted without any injury to any ligaments,muscles and facet joints of the spinal motion segment. Depending uponthe physical properties of the nucleus augmentation or replacementmedia, the outer envelope may be omitted and the media injected orplaced directly in the nucleus space. Advantageously, the presentinvention leaves the annulus fibrosis intact.

In addition, it is also possible to augment a spinal disc by introducingone or more artificial spinal disc implant or other biomaterials toprovide a functional disc replacement implant or bone growth materialsto effect fusion into the void or cavity that is made within the annulusfibrosis (AF), thereby employing the AF to retain the introducedimplants or biomaterials or fusion enhancing materials in place. The AFcan itself be used as an envelope to contain the delivered discaugmentation materials comprising spinal disc implant(s), bone growthmaterial or other biomaterials. Optionally, means are provided tocontain the disc augmentation materials within the desired space, e.g.,by delivering the disc augmentation materials into an additionalenvelope within the cavity. To effect fusion, the TASIF axial bore 152may be extended into the cephalad vertebral body and axial spinalimplants and/or bone growth material dispensed within the cavity and theTASIF axial bore 152. Or a portion of the caudal and cephaladcartilaginous endplates and vertebral body endplates can be removed inthe partial discectomy to expose vertebral bone to promote fusion withthe bone growth materials dispensed into the cavity.

As described above, the vertebroplasty procedure involves forcingvertebral body repairing or reinforcing material comprising bone growthmaterial or bone cement for repairing or reinforcing the vertebral bodyinto the cancellous bone of the fractured vertebral body. In the past,vertebroplasty has been done through a lateral exposure and penetrationof the cortical bone of a side surface of the vertebral body. As notedabove, the lateral approach presents a number of problems that areeliminated by the present invention.

One approach to performing a vertebroplasty in accordance with thepresent invention is to simply bore one or more small diameter TASIFaxial bore into the fractured cancellous bone, introduce a catheter intothe TASIF axial bore, and to pump in the bone growth material or bonecement so that it penetrates and fills the fissures in the cancellousbone. Then, the caudal end of the TASIF axial bore is closed where itpasses through the harder cortical bone forming the caudal vertebralendplate, e.g., by use of a threaded plug as described above. Thisprocedure may be repeated at several different angles.

The above-described therapies can be combined to treat a given patient.FIG. 14 illustrates, in a partial cross-section side view, a combinationof certain of the above described therapies particularly for effectingspinal fusion, and alignment, and reinforcement. In this illustratedcombined therapy, the anterior TASIF axial bore 152 is formed asdescribed above via the tract sheath 96 extending through vertebralbodies S1, L5 and into L4. Partial discectomies are performed of theintervertebral discs D5 and D4 to form respective disc cavities. A bolusof bone growth material or bone cement 60 is injected into the disccavities of intervertebral discs D4 and D5 through the axial bore.Vertebroplasty procedures are then performed to inject a bone growthmaterial or bone cement mass 94 into the gaps and fissures of cancellousbone of the two vertebral bodies L4 and L5, which may or may not befractured or collapsed. An elongated axial spinal implant 70 or 71 isinserted into the axial bore 152 and through the injected bone growthmaterial or bone cement 94 and 60 to effect a fusion with reinforcementof the vertebral bodies L4 and L5.

In this case, the elongated axial spinal implant 70 may be a 4.0 mm by60.0 mm carbon fiber rod having a spiral screw thread or otheraffixation mechanism extending along its length. This therapy mayadvantageously be performed using three such elongated axial spinalimplants implanted in this way into three diverging axial bores tomaximize the strength of the fusion.

Referring to FIG. 15, there is illustrated a spinal augmentationcatheter 200 for augmenting the vertebral bodies and/or the nucleus. Thespinal augmentation catheter 200 may be advanced along any of the axialbores described previously herein. In general, the spinal augmentationcatheter 200 is adapted to express a first media for treating and/oraugmenting a vertebral body, as well as a second, distinct media fortreating and/or augmenting the nucleus. Alternatively a single lumenaugmentation catheter such as illustrated in FIGS. 18 and 19, below, canbe used.

The spinal augmentation catheter 200 comprises an elongate rigid orflexible tubular body 206 extending between a proximal end 202 and adistal end 204. The body 206 may be manufactured in any of a variety ofmanners known in the catheter arts, such as by extrusion of suitablepolymeric materials. These include, for example, PEEK, PET, variousdensities of polyethylene, nylon, PTFE and others known in the art.Alternatively, for relatively short axial tract length treatments, thetubular body 206 may be made from a metal such as stainless steel ornitinol. Stainless steel, nitinol and other metals may also be utilizedin thin walled, flexible embodiments, as will be appreciated by those ofskill in the art in view of the disclosure herein.

The tubular body 206 preferably has a diameter which is sufficientlysmall to permit passage through the TASIF axial bore, and have asufficiently large interior cross-section to permit flow of the spinalaugmentation media. In general, the outside diameter of catheter body206 is often greater than about 0.25 inches and less than about 0.75inches. The axial length of the tubular body 206 will vary dependingupon the length of the treatment zone and the distance from the accesssite to the treatment zone.

The proximal end 202 of tubular body 206 is provided with a manifold 208as is known in the art. Manifold 208 is further provided with a firstport 210 for communicating with a first lumen 214 and, in theillustrated dual lumen embodiment, a second port 212 for communicatingwith a second lumen 216. First and second ports 210 and 212 may beprovided with any of a variety of conventional fittings, such as luerconnectors or others known in the art. Proximal manifold 208 may beinjection molded, or formed in other conventional manners.

Proximal manifold 208 may be provided with additional access ports forcommunicating with additional lumen, depending upon the desiredfunctionality of the device. For example, an additional lumen may bedesired to infuse any of a variety of fluids, medications, contrastmedia for assisting visualization, aspiration or venting functions.

First lumen 214 extends throughout the length of the tubular body 206from the proximal port 210 to a distal opening 218. In the illustratedembodiment, distal opening 218 is positioned on the distal end of thecatheter 200. Alternatively, lumen 214 may be provided with one or morelaterally opening ports, depending upon the desired performancecharacteristics.

The second lumen 216 extends between the proximal port 212 and a seconddistal opening 220. The second distal opening 220 is illustrated as alateral opening on a sidewall of the catheter, spaced apart from thefirst opening 218.

In the illustrated embodiment, the first opening 218 is spaced axiallyapart from the second opening 220 by a distance which enables the firstopening 218 to express media into the vertebral body while the secondopening 220 is positioned within the annulus fibrosa. This constructionenables augmentation of both the vertebral body and the nucleus withoutthe necessity of moving the catheter. Following expression of media intothe nucleus and vertebral body, the catheter 200 may be proximallyretracted to position the first opening 218 within the next adjacentvertebral body, for augmentation of the next vertebral body-nucleuspair. Any of a variety of augmentation media may be utilized for thevertebral body and the nucleus, as has been discussed elsewhere herein.

Additional apertures may be provided along the length of the catheterbody 206, such that at least two nucleus augmentations may besimultaneously accomplished and/or at least two vertebral bodyaugmentations may be simultaneously accomplished. For example, two orthree or more apertures in communication with the first lumen 214 may bespaced apart along the length of the catheter 200, alternating with twoor three or more apertures in communication with the second lumen 216.In this manner, a plurality of discs and vertebral bodies may beaugmented for a single axial position of the catheter 200. Followingaugmentation in the foregoing manner, the catheter 200 may be proximallyretracted from the patient, with or without leaving a stream ofaugmentation media such as by way of the first aperture 218, to fill thetract left by the catheter 200.

Alternatively, the catheter 200 may be severed or disconnected below orabout the point where it exists the spine, and left in place as apermanent implant. This can assist in maintaining the vertebral body anddisc augmentation medias in place and provide additional support for thetreated area. In an embodiment of the catheter 200 intended forpermanent implantation, the detachable component of the catheter 200 maybe similar to the implant 70 (FIG. 11) having one or more axiallyextending central lumen, in communication with the exterior through aplurality of ports. The implantable portion of the catheter 200 may berigid such as the implant 70, or may be articulated such as the implant71 (FIG. 11A). It may alternatively be flexible throughout its axiallength. The implantable portion of a catheter 200 may also be providedwith any of a variety of additional features discussed in connectionwith implant 70 or 71, such as cancellous and/or cortical bone ingrowthsurface structures, joints 79 and/or compressible shock absorbingcushions for aligning with the discs in the implanted orientation.

In all of the above-described procedures, the visualization of the spineand the introduction of instruments employed to form the anterior orposterior axial bore(s) or lumen(s) or to perform therapies, and anyspinal disc implants or axial spinal implants or other implanted medicaldevices is effected employing conventional imaging techniques includingopen MRI, fluoroscopy or ultrasound through the patient's body or usingendoscopic techniques through an axial bore. Internal visualization mayalso be desirable in any of the procedures disclosed herein. This may beaccomplished by advancing a viewing instrument such as an endoscope intothe formed bore to view the walls of the bore and/or condition of thenucleus or nucleus space prior to or during the therapeutic ordiagnostic procedure.

The various methods of accessing and treating the spine, using, forexample, the devices disclosed herein are summarized below. The methodsteps may be substituted and recombined, as will be apparent to those ofskill in the art in view of the disclosure herein, depending upon thedesired access pathway and treatment or other procedure. All patents andother publications identified above are incorporated in their entiretiesherein by reference.

Certain combined therapies are further discussed in connection withFIGS. 18 through 22. Referring to FIG. 18, there is schematicallyillustrated a first vertebral body 232 separated from a second vertebralbody 234 by a disc 236. An axial bore 230 extends through the firstvertebral body 232, the disc 236 and at least into and optionally beyondthe second vertebral body 234. An infusion catheter 238 is illustratedpositioned within the axial bore 230, for infusing a media 240 thereinas has been discussed. The nucleus has been substantially removed, withthe annulus fibrosis remaining intact.

FIG. 19 illustrates the same view as in FIG. 18, with the catheter 238proximally retracted. At this point, media 240 has been infused intoboth the second vertebral body 234 as well as the disc space and intothe first vertebral body 232.

Before, after or during infusion of media into the spine, an axialspinal implant 70 may be inserted as illustrated in FIG. 20. In theillustrated embodiment, the axial spinal implant 70 comprises a proximalend 242, a distal end 244 and a tubular body 246 extending therebetween.See FIG. 22. An axially extending lumen 248 extends throughout thelength of the implant 70. A plurality of apertures 250 through the sidewall communicate with the central lumen 248. An external thread 251 isalso provided. The proximal end 242 is provided with a releasableconnector 252, for releasable connection to a complementary connector256 on a deployment device 254.

The connectors 252 and 256 may comprise any of a variety ofcomplementary releasable connectors, which allow insertion of theimplant 70 transaxially into the treatment position in the spine, andsubsequent release from the deployment device 254. In the illustratedembodiment, the releasable connector system comprises a threadedaperture on the implant 70 for threadably engaging a threaded shaft onthe distal end of the deployment catheter 254. The connectors anddeployment catheter may be cannulated, to permit infusion of bone growthmaterial into the central lumen 248 from which it is expressed laterallythrough the apertures 250.

One or two or three or four or more implants 70 may be inserted at agiven level within the spine, depending upon the configuration anddimensions of the implant, and desired clinical performance. Referringto FIG. 21, three implants 70 are illustrated, each extending axiallythrough a unique bore. The use of two or three or more implants 70 inthis manner provides stability against rotation about the axis of anyone of the implants 70. The implants may extend generally parallel toeach other, or, as illustrated, diverge distally (cephalad) therebyenabling the use of a single introduction tract through which tointroduce each of the implants 70. Alternatively, a plurality of axiallyelongate implants may be introduced into a single axial bore, toconstruct an implant in situ which is larger in cross-section than theinside diameter of the access tract. These implants may be in the formof hollow or solid rods, having external threads and optionally otherfeatures disclosed above.

In one procedure illustrated schematically in FIGS. 18 through 21, thecatheter 238 is utilized to introduce BMP into the L5 body and theaccess tract and disc cavity. The injection catheter comprises anelongate tubular polymeric body having an outside diameter on the orderof about 4 mm, to fit through the introduction sheath. The implant 70may be in the form of a threaded solid rod or tube, or a microporouscarbon fiber rod having, e.g., a 4 mm outside diameter cross-section.Rods on the order of 4 mm diameter by 60 mm length may be utilized,although a wide variety of alternate dimensions may be utilizeddepending upon the intended clinical performance and characteristics.

Thus, in accordance with one aspect of the present invention, there isprovided a method of treating the spine. The method comprises the stepsof identifying a site on the anterior surface of the sacrum, and forminga lumen from the site through the sacrum, through a disc and into atleast one vertebrae. The site may be on the anterior side of the S2 orS1 vertebrae to accomplish, for example, anterior lumbar sacralfixations. A procedure is thereafter performed using the lumen. Theprocedure may be any of the diagnostic or therapeutic proceduresidentified above. In general, the procedure may comprise removing all orpart of a nucleus, inserting a fixation device, or inserting aprosthetic nucleus. A bone growth facilitator may also be introduced.The lumen may extend at least as far as the L5 or L4 vertebrae, inlumbar sacral fixations, and further cephalad to or through any of theL3, L2, L1 or beyond. In one embodiment, the lumen is substantiallylinear, and the forming step may comprise drilling. The lumen mayalternatively be formed into at least one disc, such as to enable asimple percutaneous discectomy.

In accordance with another aspect of the present invention, there isprovided a method which comprises the steps of identifying a site on theposterior side of the sacrum, such as on the posterior side of S2, andforming a nonlinear lumen from the site through S2 and S1, through atleast the disc at D5 and through at least one lumbar vertebrae (e.g.L5). A procedure is then performed using the lumen. The procedure may beeither a diagnostic procedure or a therapeutic procedure as discussedabove. The lumen may extend at least as far as the L4 vertebrae, andoptionally through any or each of L4, L3, L2, L1 or beyond.

In accordance with a further aspect of the present invention, there isprovided a method which comprises the steps of identifying a site on theskin of a patient, within about 10 cm from the coccyx. An access pathwayis provided at the site and through tissue to the sacrum. The accesspathway may include a minimally invasive puncture and/or a smallsurgical incision, or an open surgical cut down, although minimallyinvasive access is generally preferred. A lumen is created through thesacrum and at least one lumbar vertebrae. The lumen is thereafter usedto perform a procedure. In particular embodiments, the site on the skinof a patient is within about 5 cm, or within about 2.5 cm from thecoccyx. The lumen is generally at least about 5 cm in length, and, insome applications, is at least about 10 cm in length. The lumen may beeither linear, or curved. Generally, the lumen will extend through S1and L5, and may also extend through S2. Depending upon the desiredtreatment zone or site, the lumen may extend at least as far as each ofL4, L3, L2, L1 or beyond.

In accordance with another aspect of the present invention, there isprovided a method of treating the spine at a treatment site which isspaced apart from an access site, to minimize disruption of tissuesurrounding the treatment site. The method comprises the steps ofidentifying an access site on the spine, and advancing a device throughthe access site and into the spine. The device is further advancedaxially through the spine either inferiorly or superiorly for a distanceacross a treatment zone. The spine is treated at least in or adjacentthe treatment zone, and the distance is at least about 3 cm or at leastabout 5 cm. In some applications, the distance is at least about 7 cm or10 cm or 15 cm or greater. The access site may be on the sacrum, on thelumbar, thoracic or cervical portions of the spine. In general, thetreatment zone may include or extend beyond the second or third orfourth or fifth vertebrae in either the inferior or superior directionfrom the vertebrae which includes the access site.

The method may further comprise the step of introducing the devicepercutaneously through the skin prior to the advancing step, such asthrough a tissue pathway having a cross section no greater than about 2cm or no greater than about 1.5 cm or 1.0 cm or less. In certainapplications, the tissue pathway is within the range of from about 0.35cm to about 0.5 cm in cross section. The further advancing step maycomprise advancing the device along a linear path through the spine,typically in an anterior approach. Alternatively, the advancing step maycomprise advancing the device along a non-linear path through the spine,such as in a posterior approach to an access point on S2, or in anyapproach (anterior, posterior or lateral) to the spine in any of thelumbar, thoracic or cervical portions of the spine. The treating stepmay comprise implanting one or more fixation devices or any of thetherapeutic or diagnostic procedures discussed elsewhere herein.

In accordance with a further aspect of the present invention, there isprovided a method of treating the spine. The method comprises the stepsof creating a minimally invasive passageway through tissue and into thespine, wherein the passageway has a longitudinal axis and a length of atleast about five times its average width. The method additionallycomprises the step of introducing at least one device through thepassageway to treat the spine, wherein an extension of the axis extendsthrough at least two intervertebral discs. In certain applications, thepassageway has a length of at least about ten times its average width.The passageway may pass through the skin within about 10 cm, and in someprocedures within about 5 cm of the coccyx. In certain embodiments, thepassageway enters the spine on the anterior side. Alternatively, thepassageway may enter the spine on the posterior side.

In accordance with another aspect of the present invention, there isprovided a method of performing a procedure from the inside of thespine, while minimizing the cross-sectional area of an access pathway tothe procedure site. The method comprises the steps of advancing a devicethrough an access pathway in the spine to a procedure site, while thedevice has a first, reduced crossing profile. The pathway may have alength within the spine of at least about 3 cm or 5 cm, or as much as 10cm or 15 cm or greater, depending upon the desired access site andtreatment zone. In general, the length of the pathway is sufficient todisplace the procedure injury due to the access from the diseased orinjured hard (i.e. bone) or soft tissue at the treatment site. Thecrossing profile of at least a portion of the device is enlarged at thetreatment site to perform the procedure. The advancing step comprisesadvancing the device through at least one vertebrae and at least onedisc. The enlarging step may comprise advancing or inclining at least aportion of the device radially outwardly from a longitudinal axis of thedevice, to perform the procedure. The procedure may include any of thoseidentified elsewhere herein, including removing a portion of or all ofthe nucleus and/or implanting material into the spine.

In accordance with another aspect of the present invention, there isprovided a method of fusing the spine. The method comprises the steps ofidentifying a site on the anterior surface of the sacrum. A lumen isformed from the site through the sacrum, through a disc, and into atleast one vertebrae, and optionally through at least a second or thirdor fourth vertebrae. A fusion implant is introduced through the lumen.In one application, the introducing step comprises introducing anelongate metal fusion device. Alternatively, the introducing stepcomprises introducing a bone growth stimulating or facilitating materialor a cure in place media.

In accordance with a further aspect of the present invention, there isprovided a method of accessing the spine through a site on the anteriorof the sacrum. The method comprises the steps of introducing an accessdevice through a tissue tract from the surface of the skin to a firstsite on the anterior of the sacrum. The access device is advancedcephalad along the anterior surface of the sacrum to a second site. Thesacrum is entered at the second site. The first site may be inferior tothe S2, such as on one of the coccygeal vertebrae.

In one application, the advancing step comprises advancing the distalend of the access device, both distally and laterally, as the distal endmoves along the concavely curved anterior surface of the spine, such asthe coccygeal vertebrae or sacrum. This allows creation of a linearaccess pathway from the access point on the skin to the S2 or S1,without damaging internal structures such as the bowel which are pushedout of the way. The introducing step may comprise introducing a bluntneedle trocar to allow the device to sweep along the spine whileminimizing trauma to the spine or adjacent tissue. The introducing stepmay comprise introducing the access device through the paracoccygealspace. The second site may be on or cephalad to S2.

The method may additionally comprise a step of positioning a guide suchas a wire or a tubular sheath through the tissue tract to the secondsite. A fixation device may be introduced along the wire or through thesheath. In one application, the fixation device is positioned across atleast the S1 and L5 vertebrae. The fixation device may be positionedacross at least the S1, L5 and L4 vertebrae, and optionally into the L3vertebrae.

In accordance with yet a further aspect of the present invention, thereis provided a method of positioning an access guide such as a sheathfrom a paracoccygeal entrance point to the S2 vertebrae. The methodcomprises the steps of introducing an access device through tissue inthe paracoccygeal space. A distal end of the access device is advancedinto contact with the sacrum. The distal end is swept along the curvedanterior surface of the sacrum towards the S2 vertebrae, therebydisplacing anatomical structures such as the bowel from the path of theaccess device. The distal end of the access device is then fixed withrespect to the S2 vertebrae. The access device may be substantiallylinear or curved

The advancing step may comprise advancing the sheath through anincision. Alternatively, the advancing step may comprise advancing thesheath through a puncture. The fixing step may comprise threadablyengaging the distal end of the sheath in an aperture in the S2, ordriving a penetrating distal anchor into the bone at the fixation site.

In accordance with another aspect of the present invention, there isprovided an axial spinal implant that promotes fusion while maintainingdistraction of vertebral bodies comprising at least one spring tension,spiral spinal disc implant 160 (“spiral member”). In one embodiment,illustrated in FIGS. 23 and 24, the spiral member 160 is used inconjunction with a rod shaped spinal implant 70 to promote fusion anddistraction of vertebral bodies. The spiral member 160 can bestraightened and introduced into the spinal disc space through anintroducer tube lumen or directly through the TASIF axial bore 152. Thespiral member 160 returns to the spiral form shown in FIG. 24 uponejection into the spinal disc space 154. The cross-section 162 of thespiral member 160 can be circular or rectangular or any other suitableshape, and the spiral height is selected to fit within the disc space154 and extend between the distracted vertebral body endplates. Forexample, a rectangular cross section strip of nickel-titanium alloy(Nitinol) having a spiral shape memory can be straightened, advancedthrough the lumen of an introducer tube lumen, and released to assumethe planar spiral shape within the disc space 154.

In accordance with another aspect of the present invention, there areprovided posterior/anterior axial spinal implants 170 and rods 184 thatare adapted to be fitted into the posterior/anterior TASIF axial bores22, 152 as depicted in FIGS. 25 and 26, respectively, and therebyaddress the problems of distracting adjacent vertebrae to maintainnormal separation and shock absorption of the anterior spine,respectively. Distraction of the adjacent vertebrae may relieve pressureon nerves and relieve pain and immobility. The shock absorption ofstresses placed on the anterior spine also relieves pain and restoresmore normal anterior spine loading function. The implantation of theseTASIF axial spinal implants 170 and 184 in a TASIF axial bore 152 isdepicted in FIG. 27. It is necessary to extend the TASIF axial borethrough a spinal disc, and a disc augmentation with a toroidal or spiralartificial disc implant as described above may be performed prior toimplantation of these axial spinal implants or rods 170 and 184.

The TASIF axial spinal implant 170 illustrated in FIGS. 25 and 27comprises a threaded rod 172 extending between a caudal bore engagingbody 174 and a cephalad axial bore engaging body 176. The center portionof the threaded rod 172 may be straight as shown for insertion into ananterior TASIF axial bore 152 or curved for insertion of implant 170into a posterior TASIF axial bore 22. In use, after the cephalad axialbore engaging body 176 is fully seated into the TASIF axial bore, thecaudal bore engaging body 174 and the threaded rod are rotated withrespect to one another to cause the caudal bore engaging body 174 and acephalad axial bore engaging body 176 to separate from one another asshown by arrow 182. If the caudal bore engaging body 174 grips the TASIFaxial bore traversing the caudal vertebrae sufficiently, then the caudaland cephalad vertebrae will be distracted as the caudal bore engagingbody 174 and cephalad axial bore engaging body 176 to separate from oneanother.

This may be accomplished in several ways. As shown in FIG. 25, thethreaded rod 174 has a male threaded caudal rod end 178 that isclockwise threaded to engage the female threaded bore of the caudal boreengaging body 174 and a male threaded cephalad rod end 180 that iscounter-clockwise threaded to engage the female threaded bore of thecephalad bore engaging body 176. The caudal bore engaging body 174 isformed with exterior engaging threads or flanges 181 adapted to biteinto the caudal vertebrae by rotating it as the TASIF axial spinalimplant 170 is seated into the TASIF axial bore. The threaded rod 174can then be rotated by engagement of its caudal end and rotation with adistraction tool 183 to thereby increase the distance between the caudalbore engaging body 174 from the cephalad axial bore engaging body 176.Alternatively, cephalad end of the threaded rod 172 could be fixed tothe cephalad axial bore engaging body 176, although it would then alsobe rotated with rotation of the threaded rod 172.

The TASIF axial spinal implant 184 of FIG. 26 possesses a shockabsorbing function, two forms of which are illustrated, namely ahydraulic or a spring based, shock absorbing function. A plunger rod 186extends between a caudal bore engaging body 188 and a cephalad axialbore engaging body 190. The center portion of the plunger rod 186 may bestraight as shown for insertion into an anterior TASIF axial bore 152 orcurved for insertion of implant 170 into a posterior TASIF axial bore22. The caudal bore engaging body 188 is formed with exterior engagingthreads or flanges 196 adapted to bite into the caudal vertebrae byrotating it as the TASIF axial spinal implant 184 is seated into theTASIF axial bore. For simplicity of illustration, the caudal boreengaging body 188 is shown and described below incorporating aspring-based shock absorbing function and the cephalad axial boreengaging body 190 is shown and described below incorporating a hydraulicfluid based shock absorbing function. It will be understood that one orthe other shock absorbing function could be incorporated in a givenTASIF axial spinal implant or rod.

The caudal bore engaging body 188 is formed with an interior cavity 195retaining a spring 194 and the caudal end of the plunger rod 186. Afluid barrier gasket is also employed surrounding the caudal end of theplunger rod 186 to inhibit ingress of body fluids into the interiorcavity 195. The cephalad bore engaging body 190 is formed with aninterior cavity 192 retaining a shock absorbing fluid and the cephaladend of the plunger rod 186. A fluid barrier gasket is also employedsurrounding the cephalad end of the plunger rod 186 to inhibit ingressof body fluids into the interior cavity 192.

In use, after the cephalad axial bore engaging body 190 is fully seatedinto the TASIF axial bore, the caudal bore engaging body 188 and the rodis rotated with respect to one another using the insertion tool 199 tocause the threads 196 of the caudal bore engaging body 188 to screw intothe TASIF axial bore wall. If the caudal bore engaging body 188 gripsthe TASIF axial bore sufficiently, then the caudal and cephaladvertebrae will be distracted by the length of the plunger rod 186maintained by the spring force in a resting state. The distance betweenthe cephalad axial bore engaging body 190 and the caudal bore engagingbody 188 will shorten and lengthen in the direction of arrow 198 asforce or load is exerted between the lumbar vertebrae bodies by spinalmovement due to patient activity against spring 194 and then abates,restoring spring length. Other forms of shock absorbing mechanisms canbe employed to retain and allow the plunger rod to work against springforces and/or hydraulic forces.

The caudal and cephalad bore engaging bodies of the axial spinalimplants or rods 170 and 184 can be affixed using bone cement or one ofthe affixation mechanisms disclosed in the above-referenced provisional'748 application and '620 patent application. Bone cement materialplaced in the TASIF axial bore with the TASIF axial spinal implant curestherein such that fixation occurs between the elongated axial spinalimplant and adjacent vertebral bone of sufficient strength to withstandphysiological loads until fixation occurs by osteogenic growth betweenthe bone and the caudal and cephalad ends of the TASIF axial spinalimplant. This therapeutic procedure of the present invention can beadvantageously conducted without any injury to any ligaments, musclesand facet joints of the spinal motion segment.

The present invention described herein provides for an implantablespinal distraction/fusion rod with varied thread pitch and diameteralong different portions of its length that is capable of distractingtwo or more vertebral bodies relative to each other and/or facilitatingthe procedure of fusing the vertebral bodies together from within thespine. The present invention also provides for a method of using theabove-described rod to distract and/or fuse two or more vertebral bodiesfrom within the spine.

A distraction/fusion rod 310 according to one embodiment of the presentinvention is illustrated in FIGS. 28, 29A, and 29B. The rod 310 extendsbetween a distal, leading end 312 and a proximal, trailing end 314 andcomprises a proximal threaded section 320, a distal threaded section322, and an intermediate section 336 that can be threaded or unthreaded.In one preferred embodiment, bony fusion is bone-to-bone rather thanbeing through the inner diameter of the distraction/fusion rod 310, incontrast to spinal cages known in the art. The rod 310 is typically onthe order of 1.25 to 2.25 inches in length for a two vertebral bodyapplication. Each of the threaded sections are typically on the order of0.5 to 1.25 inches in length, whereas the intermediate section istypically on the order of 0.25 to 0.5 inch in length. It should be notedthat the actual dimensions of the rod 310 can vary depending on thephysical size and anatomical characteristics of the patient beingtreated. The rod 310 is typically produced from a biocompatiblematerial, such as, for example, titanium alloy, stainless steel,Nitinol, or various known polymers, etc., and/or from a high strengthbioabsorbable material, including, but not limited to, polymers oflactic and glycolic acid (e.g. polylactic acid, polyglycolic acid,co-polymers of these materials, bone stock, etc.).

The threaded sections 320, 322 of the rod 310 comprise coextensivecylindrical root portions 324, 326 and screw threads 328, 330. Theintermediate section 336 comprises a coextensive cylindrical rootportion 325 that may be continuous with root portion 326 and that mayhave the same outer diameter as root portion 326. In one embodiment,illustrated in FIGS. 28, 29A, and 29B, the intermediate section 336 hasa plurality of side apertures 363 that are in communication with thecentral lumen 368 of the rod 310. In another embodiment (not shown), theintermediate section 336 is threaded. In yet another embodiment (notshown), the root portion 325 of intermediate section 336 has a diameterthat is different from that of root portion 326 of the distal section322.

The root portions typically have outer diameters that are betweenapproximately 6 mm and approximately 13 mm. With continued reference tothe embodiment illustrated in FIG. 29B, the outer diameter 342 of theroot portion 324 in the proximal section 320 is greater than the outerdiameters 344, 346 of the root portions 326, 325 in the distal andintermediate sections 322, 336. In one implementation of the invention,the outer diameter 342 of the root portion 324 in the proximal section320 is approximately 9-10 mm which is larger than the outer diameters344, 346 of the root portions 326, 325 in the distal and intermediatesections 322, 336. The outer diameters 344, 346 may be approximately 6mm in the present implementation. The diameters 342, 344, 346, asillustrated in FIG. 29B, are constant along any of the three primarysections 320, 322, 336.

In another embodiment (not shown), the outer diameter of the rootportion can taper toward the leading end of the one or more of theprimary sections 320, 322, 336.

With continued reference to the embodiment shown in FIG. 29A, screwthreads 328, 330 are formed on root portions 324, 326 and extend ascontinuous threads from the trailing end to the leading end of therespective threaded sections 320, 322. The screw threads 328, 330include multiple revolutions that are spaced apart along the roots 324,326 by interthread spacings 332, 334. The proximal and distal screwthreads 328, 330 are like-handed (i.e. the threads turn in the samedirection) so that both screw threads are right-handed or so that bothare left-handed. In the embodiment illustrated in FIGS. 29A and 29B, thescrew threads 328, 330 are right-handed.

The screw threads 328, 330 are typical of “cancellous” type bone threadsknown in the art. The threads 328, 330 are typically cut with generallyflat faces on the flights of the thread with the most flat of the facesoriented in the direction of the applied load. The threads 328, 330 aretypically self-tapping screws. In one embodiment, the thread profilegenerally comprises deep flights with an asymmetric thread form, whichprovides the advantage of improved weight bearing and load distribution.The screw threads 328, 330 have both a major diameter 338, 340 and aminor diameter 341, 343. The minor diameters 341, 343 of the illustratedscrew threads 328, 330 are the same as the outer diameters 342, 344 ofroot portions 324, 326. In another embodiment, the minor diameters 341,343 are greater than the outer diameters 342, 344. In yet anotherembodiment, the minor diameters 341, 343 are less than the outerdiameters 342, 344. The major and minor diameters within any of theprimary sections 320, 322, 336 may be constant throughout the section.In another embodiment, the major and/or the minor diameters of thethreads taper from larger to smaller toward the leading end of one ormore of the primary sections 320, 322, 336.

With continued reference to the embodiment shown in FIG. 29B, the majordiameter 338 of the proximal section 320 is greater than the majordiameter 340 of the distal section 322. Similarly, the minor diameter341 of the proximal section 320 is greater than the minor diameter 343of the distal section 322. The proximal section 320 typically has amajor diameter 338 within the range of from about 10 mm to about 15 mmor greater, and often of approximately 12-13 mm, and a minor diameter341 of approximately 9-10 mm. The distal section 322 typically has amajor diameter 340 of approximately 9 mm and a minor diameter 343 ofapproximately 6 mm.

In general, the major diameter 340 of distal section 322 is smaller thanthe inside diameter of the bore into which the proximal section isintended to be threaded. In this manner, the distal section 322 canfreely axially advance through the axial bore in a proximal vertebralbody, within which the proximal section 320 is intended to be securelythreadably engaged. Since the inside diameter of the axial bore stepsdown in the area of the distal vertebral body, the distal threadedsegment 322 may be securely threadably engaged as has been discussedpreviously herein.

Thread pitch, as used herein, is defined as the distance betweencorresponding points on consecutive threads as indicated at P_(P) andP_(D) in FIG. 29A. In one embodiment, illustrated in FIGS. 28, 29A, and29B, the pitch is constant along defined sections 320, 322 of the rod,but dissimilar as compared between the proximal section 320 and thedistal section 322. Here, the pitch for a given section of the rod 310can be expressed as the number of turns along the length of a section(e.g. the number of turns per inch) even though the actual distancebetween corresponding points on consecutive threads would be calculatedby dividing the length of the section by the number of turns along thesection. With continued reference to the embodiment shown in FIG. 29A,the distal section 322 has a pitch P_(P) that is smaller than the pitchP_(D) of the proximal section 320 so that the threads 330 in the distalsection 322 are finer and closer to each other than the threads 328 inthe proximal section 320. The pitch P_(P) in the proximal section 320 istypically on the order of 10-15 turns per inch. The pitch P_(D) in thedistal section 322 is typically on the order of 10-20 turns per inch.

With continued reference to the embodiment shown in FIG. 29A, sinceP_(D) is less than P_(P), the proximal section 320 advances farther thanthe distal section 322 for any given clockwise turn applied to thetrailing end 314 of the rod 310, which in turn causes the distraction ofthe vertebral bodies engaged with the proximal and distal sections 320and 322, respectively. As mentioned previously, the embodiment shown inFIG. 29A has screw threads 328, 330 that are right-handed. As such,clockwise turns or rotations applied to the trailing end 314 of the rod310 having a finer thread pitch in the distal section 322 than in theproximal section 320 will cause the vertebral bodies engaged with theproximal and distal sections 320 and 322, respectively, to move apart orbecome distracted relative to each other. In contrast, counterclockwiseturns or rotations applied to the trailing end 314 would causecompression of the respective vertebral bodies. In another embodiment(not shown), the screw threads in the proximal and distal sections ofthe rod are left-handed, such that clockwise turns applied to thetrailing end would cause the vertebral bodies engaged with the proximaland distal sections of the rod to compress, whereas counterclockwiseturns applied to the trailing end would cause distraction of therespective vertebral bodies. In short, for a rod 310 having a finerthread pitch in the distal section 322 than in the proximal 320 section,turns applied to the trailing end 314 in the same direction as the“handedness of the rod” (i.e. clockwise turns for screws havingright-handed threading and counterclockwise turns for screws havingleft-handed threading) will cause distraction, whereas turns applied inthe opposite direction of the “handedness of the rod” (i.e.counterclockwise turns for screws having right-handed threading andclockwise turns for screws having left-handed threading) will causecontraction.

In another embodiment (not shown), the distal section 322 has a coarserthread pitch than the proximal section 320 (i.e. P_(P) is less thanP_(D)). Here, turns applied to the trailing end 314 in the samedirection as the “handedness of the rod” will cause contraction, whereasturns applied in the opposite direction of the “handedness of the rod”will cause distraction.

In another embodiment (not shown), P_(P) and P_(D) are the same, so thatthe vertebral bodies engaged with the rod do not become distracted orcompressed relative to each other. Instead, the rod 310 is advanced intothe vertebral bodies by rotating the trailing end 314 in the samedirection as the “handedness of the screw” without changing the relativepositioning of the vertebral bodies that are attached to the proximaland distal sections 320 and 322, respectively.

With continued reference to the embodiment shown in FIG. 29A, as P_(D)is decreased by increasing the number of turns in the distal sectionwhile keeping the P_(P) constant, so as to decrease P_(D) relative toP_(P). the degree of distraction caused by advancement of the rod in thedistal direction is increased. For example, if the distal section wereto have 10 turns per inch while the proximal section were to have 10turns per inch, advancement of the rod into two consecutive vertebralbodies would not change the relative position of the vertebral bodies.If, however, the distal section were to have 13 turns per inch while theproximal section were to have 10 turns per inch, the two vertebralbodies would become distracted relative to each other. For example, ifthe rod were advanced approximately 1 inch into the distal vertebralbody, the disc height (i.e. the space between the two vertebral bodies)would increase by approximately 0.3 inch. If one were to increase thenumber of turns in the distal section to 16 and then advance the rodapproximately 1 inch into the distal vertebral body, the disc heightwould increase by approximately 0.6 inch.

For a distracting embodiment, the distal pitch is therefore greater thanthe proximal pitch. In general, the distal pitch may be at least about105% of the proximal pitch. In certain applications, the distal pitch isat least about 110%, or 125% of the proximal pitch. Ratios as high as150% or more for the distal pitch may be utilized, in embodiments forwhich the degree or rate of distraction as a function of the total axialtravel of the distraction device is maximized. Specific ratios may beoptimized in view of the disclosure herein, depending upon the desiredclinical performance.

With continued reference to the embodiment illustrated in FIGS. 28 and29A, the root portion of the rod 310 extends from the distal, leadingend 312 to the proximal, trailing end 314. In one embodiment,illustrated in FIG. 30, the root portion is hollow toward the trailingend 314 and solid toward the leading end 312. For example, in oneembodiment, the root portion is hollow within the proximal section 320and solid within the intermediate and distal sections 336, 322. Theborderline between hollow and solid sections of the root portion can bevaried depending on the type of material from which the rod 310 is made,the characteristics of the male end of the drive socket, etc.

In one embodiment, the rod 310 includes a releasable coupling such as adrive socket 350 at the trailing end 314. The drive socket 350 extendsdistally and defines the lumen 368 or hollow section of the rootportion. In one embodiment, the drive socket 350 includes acircumferentially enlarged key 352 in the walls of the root portion. Aswith the depth of the hollow section of the root portion, thecharacteristics of the key 352 can be varied to accommodate thecharacteristics of the male end of the drive socket. The key 352provides a way to use a driver tool to engage the rod 310 and rotate andadvance the rod 310 distally through two or more vertebral bodies andintervening discs.

Preferably, the distal section 322 engages a first (distal) vertebralbody and the proximal section 320 engages a second vertebral bodylocated proximally relative to the first vertebral body. In oneembodiment, the device used to advance the rod 310 is a basicscrewdriver, an electromechanical device, or the like. In oneembodiment, the rod has a self-drilling tip (not illustrated) at itsleading end 312 to facilitate the advancement of the rod 310 in thedistal direction.

In one embodiment, sections of the distal section 322 and/or theintermediate section 336 are hollow. For example, in the embodimentillustrated in FIG. 31, the entire root portion (i.e. all of the primarysections 320, 322, 336) is hollow. Implantable distraction/fusion rods310 with lumens 368 or hollow sections allow for the infusion bonegrowth materials or cements and other osteogenic and/or osteoconductivematerials through and around the rods 310, and thereby facilitate theprocedure of fusing two or vertebral bodies together.

In one embodiment, illustrated in FIG. 32, the distraction/fusion rod310 has a plurality of apertures 364 in the walls of the root portion inone or more of the primary sections 320, 322, 336. The amount andlocation of apertures can be varied according to the degree ofosteogenic and/or osteoconductive material infusion desired, thematerial composition and strength (tensile, torsional, compression,etc.) of the rod, etc.

In one embodiment (not illustrated), the rod 310 contains elements thatare deployed radially after insertion of the rod in order to provideadditional rotational and/or torsional stability. The elements arepreferably integrated into the rod in a co-axial fashion and deployed bya mechanism, such as, for example, a jack screw, pull wire, etc. Theelements, when actuated, project from holes in the parent rod and extendradially outwardly from the rod into the cancellous bone of thevertebral body. Here, the elements have the added benefit of providingload sharing that would reduce the risk of subsidence of the fixedvertebral segments.

In one embodiment (not illustrated), the rod has certain features toreceive additional element that are inserted into the rod after the rodhas been positioned to engage with both the proximal and distalvertebral bodies. Here, the additional elements are configured toprovide torsional and axial load sharing ability. In another embodiment(not illustrated), the rod is coated with biologic agents that promotebone growth and healing. In yet another embodiment, the rod has aremovable microcatheter connection that allows infusion of biologicagents or drugs during the post-implant healing phase. In still anotherembodiment, the rod is constructed on a shape memory material thatprovides distracting force over time.

One method of manufacturing the distraction/fusion threaded rod 310according to the present invention is to use a computer-controlled screwmachine or lathe, as disclosed in U.S. Pat. No. 6,032,162, issued Feb.29, 2000, titled AXIAL TENSION SCREW; the entirety of this patent ishereby incorporated by reference herein and made a part of thisspecification. Variations of methods using computer-controlled screwmachines or lathes and other methods of manufacture known in the art canalso be used to manufacture the threaded rod 310.

In use, appropriately sized holes will be drilled or otherwise formedalong the long axis of the spine and centrally within the vertebralbodies being treated. The holes are of a diameter chosen to beapproximately equal to the root or minor diameter of the threads. In oneembodiment, a larger hole (i.e. the TASIF axial bore) is drilled in thelower, proximal body and a smaller hole (i.e. the extended portion ofthe TASIF axial bore) is drilled through the upper, distal vertebralbody. The drilled holes are in effect tap drills. In one embodiment, thesmaller hole is approximately ¼ inch in diameter while the larger holeis approximately ⅜ inch in diameter. In one embodiment, bone removedduring drilling operations are saved and used to fill the disc space,thus eliminating the need for a second operation to harvest bone or theneed for allograft bone or artificial bone.

After the holes are drilled and disc space between the target vertebralbodies is prepared for fusion, the rod 310 is inserted into place byturning it like a screw. If the thread pitch P_(D), P_(P) of the distaland proximal sections 322, 320 are the same, the rod will advance intoposition without modifying the relative position of the vertebrae. IfP_(D) is smaller (i.e. there are more turns per inch in the distalsection 322) than P_(P), then bodies will be separated or elevated. Thelatter embodiment is typically preferred since an affected disc space isoften collapsed, and since many therapeutic procedures for treating thespine require reestablishment of the proper disc height before applyingtreatment, such as, for example, fusion, insertion of a prostheticnucleus, etc.

One approach of using the inventive distraction/fusion rod generallycomprises: determining the desired change in disc height betweentargeted vertebral bodies; selecting a rod with the appropriate lengthand thread pitches in the distal and proximal sections to achieve thedesired change in height; accessing the targeted bodies by creating aTASIF axial bore that extends in the distal direction from the sacraltarget point or other access point to the disc space between thetargeted bodies; extending the TASIF axial bore in the distal directionto create an extended portion of the TASIF axial bore, wherein theextended portion has a smaller diameter than the portion of the TASIFaxial bore extending from the sacral target point to the disc spacebetween the targeted bodies; and advancing and implanting the selectedrod into the targeted bodies to achieve the desired change in discheight.

Determining the desired change in height involves measuring the actualor current disc height, determining the desired disc height, andsubtracting the actual disc height from the desired disc height.Measurement of disc height can be achieved through one of the manyvisualization, measurement, and/or quantification methods known to thoseskilled in the art. The desired disc height will vary depending on thephysical characteristics of the individual being treated. Typically,however, normal or pre-condition disc heights can be determined byreferring to references and standards known to those skilled in the artof treating patients for spinal conditions.

Once the desired change in disc height is determined, the next step isto select the appropriate combination of thread pitches in the distaland proximal ends to achieve the desired disc height. As explainedabove, the thread pitches P_(D), P_(P) in the distal and proximalsection of the rod can be expressed as the number of turns per a givenunit of length, such as, for example, turns/inch. As a general rule, ifthe distal section has D turns/inch and the proximal section has PWturns per inch, then placement of the rod will cause the disc height toincrease by (D−P)/P inches. For example, if the distal section has 13turns per inch while the proximal section has 10 turns per inch, theresulting increase disc height will be 3/10 inch upon full implantationof the distraction implant. If, however, the distal and proximalsections have the same number of turns per inch, then the disc heightwill not change.

After a rod with the appropriate thread pitches is selected, the nextstep is to access the targeted vertebral bodies and implant the rod.Described herein is one preferred approach to accessing the bodies andimplanting the rod. First, the patient is placed prone in the reverselithotomy position. Next, the tip of the coccyx is palpated in thesuperior gluteal fold and moved laterally and cephalad approximately 15mm. The paracoccygeal notch is palpated. A 10 mm incision located 20 mmcaudal to the notch is made and a blunt needle trocar is inserted intoincision in a “flat plane” until it impacts the transverse segment ofdistal sacrum that forms the “roof” of the paracoccygeal notch.

Next, the trocar is gently angled anteriorly to enter the paracoccygealnotch and advanced slowly to enter the presacral space. A “pop” isusually felt as the trocar traverses the fascial plane. As soon as this“pop’ is felt, advancement of the trocar is stopped and the trocar isdeflected downward to deflect tip in a posterior direction. This willcause the trocar to engage the anterior surface of the distal sacrum.Trocar position should be verified in frontal and lateral planes,particularly as it relates to the rectal air shadow. The trocar positionis maintained against the anterior sacral surface. The trocar is thegently advanced along the anterior midline of the sacrum whilecontinuously monitoring trocar advancement using biplane fluoroscopy.The trocar is advanced if the tip is against the anterior surface of thesacrum on the lateral view and in the midline on the frontal view.

In one approach, the trocar is advanced to approximately the S1/S2interspace. After, a suitable trajectory for entry into the L5/S1 discspace is determined, the trocar is rotated to position the guide hole atthe desired entry site on the sacrum, which is usually near the S1/S2interspace. Here, an inner K-wire is advanced 5 mm into the sacrum bygentle pounding. A drill motor is attached to the distal end of theK-wire and advanced distally another 5 mm. The trocar is removed anddilators are inserted to enlarge the tract. Next, a 14 mm sheath/dilatoris advanced up to the anterior surface of S1/S2. The dilator is removedand an endoscope is inserted to examine the guide entry. The veinsand/or nerves should be cauterized as necessary. If the middle sacralartery is encountered, this artery can be ligated and divided since itis typically small at this level. A sacral docking assembly is insertedthrough a 14 mm sheath and locked together at certain hubs. The sacraldocking assembly typically includes a coaxial outer sheath (12 mm innerdiameter) and an inner 7 mm cannulated bit. The distal end of the outersheath is beveled and contains 2 prongs on the heel of the bevel inorder to engage the sacrum. The outer sheath of the sacral dockingassembly can be attached to the hub of the 14 mm working sheath using aluer type hub. The inner 7 mm cannulated bit is screwed into sacrumapproximately 5 mm. Next, the 12 mm sacral docking sheath is then tappedinto sacrum. Traction is placed on cannulated bit, as necessary, toobtain proper trajectory for trans-sacral tract. The K-wire is advancedinto the sacrum across the L5/S1 disc space and into L5. Properpositioning is determined using biplane fluoroscopy. The 7 mm bit isadvanced over K-wire until it enters the central portion of the L5/S1disc. The sacral tract is enlarged as necessary.

In one approach, discectomy is then performed in the disc space. Afterthe discectomy, an endoscope is preferably used to examine disc space. Adistraction balloon is optionally inserted to gradually enlarge the discspace. Next, biologic and/or bone materials are inserted into the discspace. At this point, the self tapping distraction/fusion rod isinserted and advanced in the distal direction. After the rod is lockedinto the targeted vertebral bodies, bone chips can be used to pack therod. The final steps typically include: placing a rod cap over wire,removing the 12 mm sheath; removing the wire; re-examining the presacralspace through 14 mm sheath; removing the 14 mm sheath; and closing theincision.

As mentioned previously, a distraction rod (i.e. a rod having a finerthread pitch in its distal section relative to its proximal section)causes compression if the rod is rotated in the direction that isopposite to that of “handedness of the rod” (i.e. counterclockwise turnsapplied to a right-handed distraction rod, or clockwise turns applied toa left-handed distraction rod). In one approach to using the inventivedistraction/fusion rod, the rod is advanced in the same direction of the“handedness of the rod” until the target vertebral bodies becomeover-distracted relative to each other (i.e. deliberately spaced apartmore than desired space between the bodies). Next, the space inside andaround the rod is filled with an osteogenic agent, after which the rodis rotated in the direction that is opposite the “handedness of therod,” thereby causing compression of the target vertebral bodies. Thebodies would generally be compressed enough to compensate for excessdistraction during the stage when the rod is advanced into the vertebralbodies, thereby achieving the desired space between the bodies. Thismethod of using the distraction rod is particularly beneficial wherecompression is a prerequisite for fusion of the targeted vertebralbodies.

While the present invention has been illustrated and described withparticularity in terms of preferred embodiments, it should be understoodthat no limitation of the scope of the invention is intended thereby.Features of any of the foregoing methods and devices may be substitutedor added into the others, as will be apparent to those of skill in theart. The scope of the invention is defined only by the claims appendedhereto. It should also be understood that variations of the particularembodiments described herein incorporating the principles of the presentinvention will occur to those of ordinary skill in the art and yet bewithin the scope of the appended claims.

1. An axially-extending implantable spinal fusion rod for adjusting adistance between two adjacent vertebral bodies in a spine, said rodcomprising: an elongated body dimensioned for implantation along an axisextending from an access point on the anterior surface of the sacrum andfor maintaining the distance between the two adjacent vertebral bodies;a first section near a leading end of the rod that has a first threadingfor engaging with a first vertebral body, the first threading comprisinga first major diameter and a first minor diameter; a second section neara trailing end of the rod that has a second threading for engaging witha second vertebral body that is located inferiorly relative to the firstvertebral body, the second threading comprising a second major diameterand a second minor diameter; and an intermediate section extendingbetween the distal and proximal sections, the intermediate section beingsized and configured for maintaining the distance between the twovertebral bodies; wherein the first major diameter of the firstthreading is less than the second major diameter of the secondthreading; wherein a first thread pitch in the first section is finerrelative to a second thread pitch in the second section such thatadvancement of the first section distally into the first vertebral bodyby rotation causes an increase in distance between the first and secondvertebral bodies and wherein a ratio of the axial length of the secondthreading to the second minor diameter is within the range ofapproximately 1.27 to 3.53.
 2. The implantable rod according to claim 1,wherein the second major diameter is within the range of from about 10mm to about 15 mm.
 3. The implantable rod according to claim 1, whereinthe first major diameter is no greater than about 98% of the secondmajor diameter.
 4. The implantable rod according to claim 1, wherein theelongated body has a length within the range of from about 1.25 inchesto about 2.25 inches.
 5. The implantable rod according to claim 1,wherein each of the first threading and the second threading has anaxial length within the range of from about 0.5 inches to about 1.25inches.
 6. The implantable rod according to claim 1, wherein theintermediate section has an axial length within the range of from about0.25 inches to about 0.5 inches.
 7. The implantable rod according toclaim 1, wherein the elongated body comprises stainless steel.
 8. Theimplantable rod according to claim 1, wherein the elongated bodycomprises titanium.
 9. The implantable rod according to claim 1, whereinthe elongated body comprises a bioabsorbable material.
 10. Theimplantable rod according to claim 1, wherein each of the firstthreading and the second threading extends counterclockwise around thebody.
 11. The implantable rod according to claim 1, wherein each of thefirst threading and the second threading extends clockwise around thebody.
 12. The implantable rod according to claim 1, wherein each of thefirst threading and the second threading are self tapping.
 13. Theimplantable rod according to claim 1, wherein the second major diameteris within the range of from about 12 mm to about 13 mm.
 14. Theimplantable rod according to claim 1, wherein the first major diameteris about 9 mm.
 15. The implantable rod according to claim 1, wherein thesecond thread pitch is within the range of from about 10 to about 15turns per inch.
 16. The implantable rod according to claim 1, whereinthe first thread pitch is within the range of from about 10 to about 20turns per inch.
 17. The implantable rod according to claim 1, whereinthe first thread pitch is at least about 105% finer than the secondthread pitch.
 18. The implantable rod according to claim 1, wherein thefirst thread pitch is at least about 110% finer than the second threadpitch.
 19. The implantable rod according to claim 1, wherein the firstthread pitch is at least about 125% finer than the second thread pitch.20. The implantable rod according to claim 1, further comprising areleasable coupling, for releasably engaging a rotatable driver tool.21. An axially-extending implantable spinal fusion rod for adjusting adistance between two adjacent vertebral bodies in a spine, said rodcomprising: an elongated body dimensioned for implantation along an axisextending from an access point on the anterior surface of the sacrum andfor maintaining the distance between the two adjacent vertebral bodies;a first section near a leading end of the rod that has a first threadingfor engaging with a first vertebral body, the first threading comprisinga first major diameter and a first minor diameter; a second section neara trailing end of the rod that has a second threading for engaging witha second vertebral body that is located inferiorly relative to the firstvertebral body, the second threading comprising a second major diameterand a second minor diameter; and an intermediate section extendingbetween the distal and proximal sections, the intermediate section beingsized and configured for maintaining the distance between the twovertebral bodies; wherein the first major diameter of the firstthreading is less than the second major diameter of the secondthreading; wherein a first thread pitch in the first section is finerrelative to a second thread pitch in the second section such thatadvancement of the first section distally into the first vertebral bodyby rotation causes an increase in distance between the first and secondvertebral bodies and wherein the first section further comprises a firstminor diameter, and a ratio of the axial length of the first threadingto the first minor diameter is within the range of approximately 2.12 to5.29.
 22. The implantable rod according to claim 21, wherein the secondmajor diameter is within the range of from about 10 mm to about 15 mm.23. The implantable rod according to claim 21, wherein the first majordiameter is no greater than about 98% of the second major diameter. 24.The implantable rod according to claim 21, wherein the elongated bodyhas a length within the range of from about 1.25 inches to about 2.25inches.
 25. The implantable rod according to claim 21, wherein each ofthe first threading and the second threading has an axial length withinthe range of from about 0.5 inches to about 1.25 inches.
 26. Theimplantable rod according to claim 21, wherein the intermediate sectionhas an axial length within the range of from about 0.25 inches to about0.5 inches.
 27. An axially-extending implantable spinal fusion rod formaintaining a distance between two adjacent vertebral bodies in a spine,said rod comprising: an elongated body dimensioned for implantationalong an axis extending from an access point on the anterior surface ofthe sacrum and for maintaining the distance between the two adjacentvertebral bodies; a first section near a leading end of the rod that hasa first threading for engaging with a first vertebral body, the firstthreading comprising a first major diameter and a first minor diameter;a second section near a trailing end of the rod that has a secondthreading for engaging with a second vertebral body that is locatedinferiorly relative to the first vertebral body, the second threadingcomprising a second major diameter and a second minor diameter; and anintermediate section extending between the distal and proximal sections,the intermediate section being sized and configured for maintaining thedistance between the two vertebral bodies; wherein the first majordiameter of the first threading is less than the second major diameterof the second threading; a ratio of the axial length of the secondthreading to the second minor diameter is within the range ofapproximately 1.27 to 3.53.
 28. The implantable rod according to claim27, wherein the second major diameter is within the range of from about10 mm to about 15 mm.
 29. The implantable rod according to claim 27,wherein the first major diameter is no greater than about 98% of thesecond major diameter.
 30. The implantable rod according to claim 27,wherein the elongated body has a length within the range of from about1.25 inches to about 2.25 inches.
 31. The implantable rod according toclaim 27, wherein each of the first threading and the second threadinghas an axial length within the range of from about 0.5 inches to about1.25 inches.
 32. The implantable rod according to claim 27, wherein theintermediate section has an axial length within the range of from about0.25 inches to about 0.5 inches.
 33. The implantable rod according toclaim 27, wherein the elongated body comprises stainless steel.
 34. Theimplantable rod according to claim 27, wherein the elongated bodycomprises titanium.
 35. The implantable rod according to claim 27,wherein the elongated body comprises a bioabsorbable material.